diff --git a/app/components/settings/time_zone_setting_component.rb b/app/components/settings/time_zone_setting_component.rb
index 674ad6a53ce1..772c8e35b012 100644
--- a/app/components/settings/time_zone_setting_component.rb
+++ b/app/components/settings/time_zone_setting_component.rb
@@ -30,7 +30,7 @@
 
 module Settings
   ##
-  # A text field to enter numeric values.
+  # A select field to select a time zone from.
   class TimeZoneSettingComponent < ::ApplicationComponent
     options :form, :title
     options container_class: "-wide"
diff --git a/app/models/anonymous_user.rb b/app/models/anonymous_user.rb
index 27e4514d6563..b55057d0e928 100644
--- a/app/models/anonymous_user.rb
+++ b/app/models/anonymous_user.rb
@@ -27,31 +27,25 @@
 #++
 
 class AnonymousUser < User
-  validate :validate_unique_anonymous_user, on: :create
-
-  # There should be only one AnonymousUser in the database
-  def validate_unique_anonymous_user
-    errors.add :base, "An anonymous user already exists." if AnonymousUser.any?
-  end
-
-  def available_custom_fields
-    []
-  end
-
-  # Overrides a few properties
-  def logged?; false end
-
-  def builtin?; true end
-
-  def admin; false end
+  include Users::FunctionUser
 
   def name(*_args); I18n.t(:label_user_anonymous) end
 
-  def mail; nil end
+  def self.first
+    anonymous_user = super
 
-  def time_zone; nil end
+    if anonymous_user.nil?
+      (anonymous_user = new.tap do |u|
+        u.lastname = "Anonymous"
+        u.login = ""
+        u.firstname = ""
+        u.mail = ""
+        u.status = User.statuses[:active]
+      end).save
 
-  def rss_key; nil end
+      raise "Unable to create the anonymous user." if anonymous_user.new_record?
+    end
 
-  def destroy; false end
+    anonymous_user
+  end
 end
diff --git a/app/models/deleted_user.rb b/app/models/deleted_user.rb
index ae65cfb4af70..ad89f12e71e0 100644
--- a/app/models/deleted_user.rb
+++ b/app/models/deleted_user.rb
@@ -1,23 +1,9 @@
 class DeletedUser < User
-  validate :validate_unique_deleted_user, on: :create
-
-  # There should be only one DeletedUser in the database
-  def validate_unique_deleted_user
-    errors.add :base, "A DeletedUser already exists." if DeletedUser.any?
-  end
-
   def self.first
     super || create(type: to_s, status: statuses[:locked])
   end
 
-  # Overrides a few properties
-  def available_custom_fields = []
-  def logged? = false
-  def builtin? = true
-  def admin = false
+  include Users::FunctionUser
+
   def name(*_args) = I18n.t("user.deleted")
-  def mail = nil
-  def time_zone = nil
-  def rss_key = nil
-  def destroy = false
 end
diff --git a/app/models/exports/concerns/csv.rb b/app/models/exports/concerns/csv.rb
index 859e6e0ccaa6..5c04f1b64d9c 100644
--- a/app/models/exports/concerns/csv.rb
+++ b/app/models/exports/concerns/csv.rb
@@ -84,7 +84,7 @@ def format_csv(record, attribute)
       def csv_export_filename
         sane_filename(
           "#{Setting.app_title} #{title} \
-          #{format_time_as_date(Time.zone.now, '%Y-%m-%d')}.csv"
+          #{format_time_as_date(Time.zone.now, format: '%Y-%m-%d')}.csv"
         )
       end
     end
diff --git a/app/models/system_user.rb b/app/models/system_user.rb
index 1a3195dacc4b..a840ef477bbd 100644
--- a/app/models/system_user.rb
+++ b/app/models/system_user.rb
@@ -31,31 +31,35 @@
 #
 
 class SystemUser < User
-  validate :validate_unique_system_user, on: :create
-
-  # There should be only one SystemUser in the database
-  def validate_unique_system_user
-    errors.add :base, "A SystemUser already exists." if SystemUser.any?
-  end
-
-  # Overrides a few properties
-  def logged?; false end
-
-  def builtin?; true end
+  include Users::FunctionUser
 
   def name(*_args); "System" end
 
-  def mail; nil end
+  def run_given
+    User.execute_as(self) do
+      yield self
+    end
+  end
 
-  def time_zone; nil end
+  def self.first
+    system_user = super
 
-  def rss_key; nil end
+    if system_user.nil?
+      system_user = new(
+        firstname: "",
+        lastname: "System",
+        login: "",
+        mail: "",
+        admin: true,
+        status: User.statuses[:active],
+        first_login: false
+      )
 
-  def destroy; false end
+      system_user.save
 
-  def run_given
-    User.execute_as(self) do
-      yield self
+      raise "Unable to create the system user." unless system_user.persisted?
     end
+
+    system_user
   end
 end
diff --git a/app/models/user.rb b/app/models/user.rb
index 577ee7606183..11749506f856 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -399,7 +399,7 @@ def log_failed_login
   end
 
   def log_successful_login
-    update_attribute(:last_login_on, Time.now)
+    update_attribute(:last_login_on, Time.current)
   end
 
   def pref
@@ -407,7 +407,13 @@ def pref
   end
 
   def time_zone
-    @time_zone ||= (pref.time_zone.blank? ? nil : ActiveSupport::TimeZone[pref.time_zone])
+    @time_zone ||= ActiveSupport::TimeZone[pref.time_zone] || ActiveSupport::TimeZone["Etc/UTC"]
+  end
+
+  def reload(*)
+    @time_zone = nil
+
+    super
   end
 
   def wants_comments_in_reverse_order?
@@ -538,46 +544,12 @@ def missing_authentication_method?
 
   # Returns the anonymous user.  If the anonymous user does not exist, it is created.  There can be only
   # one anonymous user per database.
-  def self.anonymous # rubocop:disable Metrics/AbcSize
-    RequestStore[:anonymous_user] ||=
-      begin
-        anonymous_user = AnonymousUser.first
-
-        if anonymous_user.nil?
-          (anonymous_user = AnonymousUser.new.tap do |u|
-            u.lastname = "Anonymous"
-            u.login = ""
-            u.firstname = ""
-            u.mail = ""
-            u.status = User.statuses[:active]
-          end).save
-
-          raise "Unable to create the anonymous user." if anonymous_user.new_record?
-        end
-        anonymous_user
-      end
+  def self.anonymous
+    RequestStore[:anonymous_user] ||= AnonymousUser.first
   end
 
   def self.system
-    system_user = SystemUser.first
-
-    if system_user.nil?
-      system_user = SystemUser.new(
-        firstname: "",
-        lastname: "System",
-        login: "",
-        mail: "",
-        admin: true,
-        status: User.statuses[:active],
-        first_login: false
-      )
-
-      system_user.save(validate: false)
-
-      raise "Unable to create the automatic migration user." unless system_user.persisted?
-    end
-
-    system_user
+    SystemUser.first
   end
 
   protected
@@ -689,6 +661,6 @@ def log_failed_login_timestamp
   end
 
   def self.default_admin_account_changed?
-    !User.active.find_by_login("admin").try(:current_password).try(:matches_plaintext?, "admin") # rubocop:disable Rails/DynamicFindBy
+    !User.active.find_by_login("admin").try(:current_password).try(:matches_plaintext?, "admin")
   end
 end
diff --git a/app/models/user_preference.rb b/app/models/user_preference.rb
index a2f49ede552a..220768c6a2df 100644
--- a/app/models/user_preference.rb
+++ b/app/models/user_preference.rb
@@ -132,7 +132,11 @@ def high_contrast_theme?
   end
 
   def time_zone
-    super.presence || Setting.user_default_timezone.presence
+    super.presence || Setting.user_default_timezone.presence || "Etc/UTC"
+  end
+
+  def time_zone?
+    settings["time_zone"].present?
   end
 
   def daily_reminders
diff --git a/app/models/users/function_user.rb b/app/models/users/function_user.rb
new file mode 100644
index 000000000000..fe8309243f53
--- /dev/null
+++ b/app/models/users/function_user.rb
@@ -0,0 +1,56 @@
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+module Users::FunctionUser
+  extend ActiveSupport::Concern
+
+  included do
+    validate :validate_unique_function_user, on: :create
+
+    # There should be only one such user in the database
+    def validate_unique_function_user
+      errors.add :base, "A #{self.class.name} already exists." if self.class.any?
+    end
+
+    def available_custom_fields = []
+
+    def logged? = false
+
+    def builtin? = true
+
+    def name(*_args); raise NotImplementedError end
+
+    def mail = nil
+
+    def time_zone; ActiveSupport::TimeZone[Setting.user_default_timezone.presence || "Etc/UTC"] end
+
+    def rss_key = nil
+
+    def destroy = false
+  end
+end
diff --git a/app/models/work_package/pdf_export/page.rb b/app/models/work_package/pdf_export/page.rb
index ff5f1eea866b..571efcfaf76f 100644
--- a/app/models/work_package/pdf_export/page.rb
+++ b/app/models/work_package/pdf_export/page.rb
@@ -127,7 +127,7 @@ def footer_page_nr
   end
 
   def footer_date
-    format_time(Time.zone.now, true)
+    format_time(Time.zone.now)
   end
 
   def total_page_nr_text
diff --git a/app/views/admin/settings/users_settings/show.html.erb b/app/views/admin/settings/users_settings/show.html.erb
index 0f67f8b37049..b4171880d12e 100644
--- a/app/views/admin/settings/users_settings/show.html.erb
+++ b/app/views/admin/settings/users_settings/show.html.erb
@@ -48,7 +48,6 @@ See COPYRIGHT and LICENSE files for more details.
 
     <%= render Settings::TimeZoneSettingComponent.new(
           "user_default_timezone",
-          container_class: "-slim",
           title: I18n.t("tooltip_user_default_timezone")
         )
     %>
diff --git a/app/views/users/_preferences.html.erb b/app/views/users/_preferences.html.erb
index 44a513b0527e..37d8fe87686e 100644
--- a/app/views/users/_preferences.html.erb
+++ b/app/views/users/_preferences.html.erb
@@ -30,6 +30,7 @@ See COPYRIGHT and LICENSE files for more details.
   <%= render Settings::TimeZoneSettingComponent.new(
     "time_zone",
     form: pref_fields,
+    include_blank: false,
     container_class: (defined? input_size) ? "-#{input_size}" : "-wide"
   )
   %>
diff --git a/lib_static/redmine/i18n.rb b/lib_static/redmine/i18n.rb
index 01179d7dd3b1..243027c58fbd 100644
--- a/lib_static/redmine/i18n.rb
+++ b/lib_static/redmine/i18n.rb
@@ -115,18 +115,18 @@ def link_regex
       /(\[(.+?)\]\((.+?)\))/
     end
 
-    # Format the time to a date in the user time zone if one is set.
-    # If none is set and the time is in utc time zone (meaning it came from active record), format the date in the system timezone
-    # otherwise just use the date in the time zone attached to the time.
-    def format_time_as_date(time, format = nil)
+    # Formats the given time as a date string according to the user's time zone and
+    # optional specified format.
+    #
+    # @param time [Time] The time to format.
+    # @param format [String, nil] The strftime format to use for the date. If nil, the default
+    #   date format from `Setting.date_format` is used.
+    # @return [String, nil] The formatted date string, or nil if the time is not provided.
+    def format_time_as_date(time, format: nil)
       return nil unless time
 
       zone = User.current.time_zone
-      local_date = (if zone
-                      time.in_time_zone(zone)
-                    else
-                      time.utc? ? time.localtime : time
-                    end).to_date
+      local_date = time.in_time_zone(zone).to_date
 
       if format
         local_date.strftime(format)
@@ -135,18 +135,34 @@ def format_time_as_date(time, format = nil)
       end
     end
 
-    def format_time(time, include_date = true)
+    # Formats the given time as a time string according to the user's time zone
+    # and optional specified format.
+    #
+    # @param time [Time] The time to format.
+    # @param include_date [Boolean] Whether to include the date in the formatted
+    #   output. Defaults to true.
+    # @param format [String] The strftime format to use for the time. Defaults
+    #   to the format in `Setting.time_format`.
+    # @return [String, nil] The formatted time string, or nil if the time is not
+    #   provided.
+    def format_time(time, include_date: true, format: Setting.time_format)
       return nil unless time
 
-      time = time.to_time if time.is_a?(String)
       zone = User.current.time_zone
-      local = if zone
-                time.in_time_zone(zone)
-              else
-                (time.utc? ? time.to_time.localtime : time)
-              end
+      local = time.in_time_zone(zone)
+
       (include_date ? "#{format_date(local)} " : "") +
-        (Setting.time_format.blank? ? ::I18n.l(local, format: :time) : local.strftime(Setting.time_format))
+        (format.blank? ? ::I18n.l(local, format: :time) : local.strftime(format))
+    end
+
+    # Returns the offset to UTC (with utc prepended) currently active
+    # in the current users time zone. DST is factored in so the offset can
+    # shift over the course of the year
+    def formatted_time_zone_offset
+      # Doing User.current.time_zone and format that will not take heed of DST as it has no notion
+      # of a current time.
+      # https://github.com/rails/rails/issues/7297
+      "UTC#{User.current.time_zone.now.formatted_offset}"
     end
 
     def day_name(day)
diff --git a/modules/bim/lib/open_project/bim/bcf_xml/exporter.rb b/modules/bim/lib/open_project/bim/bcf_xml/exporter.rb
index b3dbb206a22d..dcb27d4b8d64 100644
--- a/modules/bim/lib/open_project/bim/bcf_xml/exporter.rb
+++ b/modules/bim/lib/open_project/bim/bcf_xml/exporter.rb
@@ -43,7 +43,7 @@ def bcf_filename
 
       sane_filename(
         "#{Setting.app_title} #{I18n.t(:label_work_package_plural)} \
-        #{format_time_as_date(Time.now, '%Y-%m-%d')}.bcf"
+        #{format_time_as_date(Time.current, format: '%Y-%m-%d')}.bcf"
       )
     end
 
diff --git a/modules/boards/app/components/boards/row_component.rb b/modules/boards/app/components/boards/row_component.rb
index e774c1fa3f38..7323c5140470 100644
--- a/modules/boards/app/components/boards/row_component.rb
+++ b/modules/boards/app/components/boards/row_component.rb
@@ -39,7 +39,7 @@ def name
     end
 
     def created_at
-      safe_join([helpers.format_date(model.created_at), helpers.format_time(model.created_at, false)], " ")
+      safe_join([helpers.format_date(model.created_at), helpers.format_time(model.created_at, include_date: false)], " ")
     end
 
     def type
diff --git a/modules/meeting/app/components/meetings/row_component.rb b/modules/meeting/app/components/meetings/row_component.rb
index 89b578bed95e..b945f6ddf157 100644
--- a/modules/meeting/app/components/meetings/row_component.rb
+++ b/modules/meeting/app/components/meetings/row_component.rb
@@ -47,7 +47,7 @@ def type
     end
 
     def start_time
-      safe_join([helpers.format_date(model.start_time), helpers.format_time(model.start_time, false)], " ")
+      safe_join([helpers.format_date(model.start_time), helpers.format_time(model.start_time, include_date: false)], " ")
     end
 
     def duration
diff --git a/modules/meeting/app/components/meetings/side_panel/details_component.html.erb b/modules/meeting/app/components/meetings/side_panel/details_component.html.erb
index a49740fe66e1..1c55629c1ce8 100644
--- a/modules/meeting/app/components/meetings/side_panel/details_component.html.erb
+++ b/modules/meeting/app/components/meetings/side_panel/details_component.html.erb
@@ -30,13 +30,13 @@
             flex_layout(align_items: :center) do |time|
               time.with_column do
                 render(Primer::Beta::Text.new) do
-                  "#{format_time(@meeting.start_time, false)} - #{format_time(@meeting.end_time, false)}"
+                  "#{format_time(@meeting.start_time, include_date: false)} - #{format_time(@meeting.end_time, include_date:false)}"
                 end
               end
 
               time.with_column(ml: 2) do
                 render(Primer::Beta::Text.new(color: :subtle, font_size: :small)) do
-                  (User.current.time_zone || Time.zone).to_s[/\((.*?)\)/m, 1]
+                  formatted_time_zone_offset
                 end
               end
             end
diff --git a/modules/meeting/app/components/meetings/side_panel/details_form_component.rb b/modules/meeting/app/components/meetings/side_panel/details_form_component.rb
index e290cdbe3e72..fb0bdde00cdb 100644
--- a/modules/meeting/app/components/meetings/side_panel/details_form_component.rb
+++ b/modules/meeting/app/components/meetings/side_panel/details_form_component.rb
@@ -45,11 +45,11 @@ def render?
     private
 
     def start_date_initial_value
-      @meeting.start_time&.strftime("%Y-%m-%d")
+      format_time_as_date(@meeting.start_time, format: "%Y-%m-%d")
     end
 
     def start_time_initial_value
-      @meeting.start_time&.strftime("%H:%M")
+      format_time(@meeting.start_time, include_date: false, format: "%H:%M")
     end
   end
 end
diff --git a/modules/meeting/app/controllers/meetings_controller.rb b/modules/meeting/app/controllers/meetings_controller.rb
index c83418b239c5..5fb9334eb5c6 100644
--- a/modules/meeting/app/controllers/meetings_controller.rb
+++ b/modules/meeting/app/controllers/meetings_controller.rb
@@ -27,7 +27,6 @@
 #++
 
 class MeetingsController < ApplicationController
-  around_action :set_time_zone
   before_action :load_and_authorize_in_optional_project, only: %i[index new show create history]
   before_action :verify_activities_module_activated, only: %i[history]
   before_action :determine_date_range, only: %i[history]
@@ -96,8 +95,8 @@ def create # rubocop:disable Metrics/AbcSize
 
     if call.success?
       text = I18n.t(:notice_successful_create)
-      if User.current.time_zone.nil?
-        link = I18n.t(:notice_timezone_missing, zone: Time.zone)
+      unless User.current.pref.time_zone?
+        link = I18n.t(:notice_timezone_missing, zone: formatted_time_zone_offset)
         text += " #{view_context.link_to(link, { controller: '/my', action: :settings, anchor: 'pref_time_zone' },
                                          class: 'link_to_profile')}"
       end
@@ -288,17 +287,6 @@ def load_meetings(query)
       .paginate(page: page_param, per_page: per_page_param)
   end
 
-  def set_time_zone(&)
-    zone = User.current.time_zone
-    if zone.nil?
-      localzone = Time.current.utc_offset
-      localzone -= 3600 if Time.current.dst?
-      zone = ::ActiveSupport::TimeZone[localzone]
-    end
-
-    Time.use_zone(zone, &)
-  end
-
   def build_meeting
     @meeting = Meeting.new
     @meeting.project = @project
diff --git a/modules/meeting/app/forms/meeting/start_time.rb b/modules/meeting/app/forms/meeting/start_time.rb
index d077e40230f0..36b43092839e 100644
--- a/modules/meeting/app/forms/meeting/start_time.rb
+++ b/modules/meeting/app/forms/meeting/start_time.rb
@@ -27,6 +27,8 @@
 #++
 
 class Meeting::StartTime < ApplicationForm
+  include Redmine::I18n
+
   form do |meeting_form|
     meeting_form.text_field(
       name: :start_time_hour,
@@ -36,7 +38,7 @@ class Meeting::StartTime < ApplicationForm
       label: Meeting.human_attribute_name(:start_time),
       leading_visual: { icon: :clock },
       required: true,
-      caption: Time.zone.to_s[/\((.*?)\)/m, 1]
+      caption: formatted_time_zone_offset
     )
   end
 
diff --git a/modules/meeting/app/forms/meeting_agenda_item/meeting_form.rb b/modules/meeting/app/forms/meeting_agenda_item/meeting_form.rb
index 976f4ca5580c..cd44d1fedb88 100644
--- a/modules/meeting/app/forms/meeting_agenda_item/meeting_form.rb
+++ b/modules/meeting/app/forms/meeting_agenda_item/meeting_form.rb
@@ -52,7 +52,7 @@ class MeetingAgendaItem::MeetingForm < ApplicationForm
             label: "#{meeting.project.name}: " \
                    "#{meeting.title} " \
                    "#{format_date(meeting.start_time)} " \
-                   "#{format_time(meeting.start_time, false)}",
+                   "#{format_time(meeting.start_time, include_date: false)}",
             value: meeting.id
           )
         end
diff --git a/modules/meeting/app/mailers/meeting_mailer.rb b/modules/meeting/app/mailers/meeting_mailer.rb
index 817962f41c3d..fa026b3eb582 100644
--- a/modules/meeting/app/mailers/meeting_mailer.rb
+++ b/modules/meeting/app/mailers/meeting_mailer.rb
@@ -63,8 +63,6 @@ def icalendar_notification(meeting, user, _actor, **)
     set_headers @meeting
 
     with_attached_ics(meeting, user) do
-      timezone = Time.zone || Time.zone_default
-      @formatted_timezone = format_timezone_offset timezone, @meeting.start_time
       subject = "[#{@meeting.project.name}] #{@meeting.title}"
       mail(to: user, subject:)
     end
@@ -95,9 +93,4 @@ def set_headers(meeting)
     headers["Content-Type"] = 'text/calendar; charset=utf-8; method="PUBLISH"; name="meeting.ics"'
     headers["Content-Transfer-Encoding"] = "8bit"
   end
-
-  def format_timezone_offset(timezone, time)
-    offset = ::ActiveSupport::TimeZone.seconds_to_utc_offset time.utc_offest_for_timezone(timezone), true
-    "(GMT#{offset}) #{timezone.name}"
-  end
 end
diff --git a/modules/meeting/app/models/activities/meeting_activity_provider.rb b/modules/meeting/app/models/activities/meeting_activity_provider.rb
index 802eb0739452..01003beee95f 100644
--- a/modules/meeting/app/models/activities/meeting_activity_provider.rb
+++ b/modules/meeting/app/models/activities/meeting_activity_provider.rb
@@ -109,8 +109,8 @@ def event_title(event)
       end_time = start_time + event["meeting_duration"].to_f.hours
 
       fstart_with = format_date start_time
-      fstart_without = format_time start_time, false
-      fend_without = format_time end_time, false
+      fstart_without = format_time start_time, include_date: false
+      fend_without = format_time end_time, include_date: false
 
       "#{I18n.t(:label_meeting)}: #{event['meeting_title']} (#{fstart_with} #{fstart_without}-#{fend_without})"
     else
diff --git a/modules/meeting/app/models/activities/meeting_event_mapper.rb b/modules/meeting/app/models/activities/meeting_event_mapper.rb
index a02cb4c6ad31..281839a5390a 100644
--- a/modules/meeting/app/models/activities/meeting_event_mapper.rb
+++ b/modules/meeting/app/models/activities/meeting_event_mapper.rb
@@ -204,8 +204,8 @@ def event_title(_journal, data)
     end_time = start_time + data[:meeting_duration].to_f.hours
 
     fstart_with = format_date start_time
-    fstart_without = format_time start_time, false
-    fend_without = format_time end_time, false
+    fstart_without = format_time start_time, include_date: false
+    fend_without = format_time end_time, include_date: false
 
     "#{I18n.t(:label_meeting)}: #{data[:meeting_title]} (#{fstart_with} #{fstart_without}-#{fend_without})"
   end
diff --git a/modules/meeting/app/models/meeting.rb b/modules/meeting/app/models/meeting.rb
index 58b362589eb6..27697a97fc57 100644
--- a/modules/meeting/app/models/meeting.rb
+++ b/modules/meeting/app/models/meeting.rb
@@ -253,14 +253,15 @@ def allowed_participants
 
   def set_initial_values
     # set defaults
-    write_attribute(:start_time, Date.tomorrow + 10.hours) if start_time.nil?
+    # Start date is set to tomorrow at 10 AM (Current users local time)
+    write_attribute(:start_time, User.current.time_zone.now.at_midnight + 34.hours) if start_time.nil?
     self.duration ||= 1
     update_derived_fields
   end
 
   def update_derived_fields
-    @start_date = start_time.to_date.iso8601
-    @start_time_hour = start_time.strftime("%H:%M")
+    @start_date = format_time_as_date(start_time, format: "%Y-%m-%d")
+    @start_time_hour = format_time(start_time, include_date: false, format: "%H:%M")
   end
 
   private
diff --git a/modules/meeting/app/models/meeting/journalized.rb b/modules/meeting/app/models/meeting/journalized.rb
index cd49c4211119..b26ffc7b5cb0 100644
--- a/modules/meeting/app/models/meeting/journalized.rb
+++ b/modules/meeting/app/models/meeting/journalized.rb
@@ -35,7 +35,7 @@ module Meeting::Journalized
     acts_as_event title: Proc.new { |o|
                            "#{I18n.t(:label_meeting)}: #{o.title} \
           #{format_date o.start_time} \
-          #{format_time o.start_time, false}-#{format_time o.end_time, false})"
+          #{format_time o.start_time, include_date: false}-#{format_time o.end_time, include_date: false})"
                          },
                   url: Proc.new { |o| { controller: "/meetings", action: "show", id: o } },
                   author: Proc.new(&:user),
diff --git a/modules/meeting/app/views/meeting_mailer/icalendar_notification.html.erb b/modules/meeting/app/views/meeting_mailer/icalendar_notification.html.erb
index 1aea9386abca..4c2129253fb3 100644
--- a/modules/meeting/app/views/meeting_mailer/icalendar_notification.html.erb
+++ b/modules/meeting/app/views/meeting_mailer/icalendar_notification.html.erb
@@ -4,7 +4,7 @@
 <p><%= t(:text_notificiation_invited) %></p>
 
 <ul>
-<li><%=t :label_meeting_date_time %>: <%= format_time_as_date @meeting.start_time %> <%= format_time @meeting.start_time, false %>-<%= format_time @meeting.end_time, false %> <%= @formatted_timezone %></li>
+<li><%=t :label_meeting_date_time %>: <%= format_time_as_date @meeting.start_time %> <%= format_time @meeting.start_time, include_date: false %>-<%= format_time @meeting.end_time, include_date: false %> (<%= formatted_time_zone_offset %>)</li>
 <li><%=Meeting.human_attribute_name(:location) %>: <%= @meeting.location %></li>
 <li><%=Meeting.human_attribute_name(:participants_invited) %>: <%= @meeting.participants.invited.sort.join("; ") %></li>
 <li><%=Meeting.human_attribute_name(:participants_attended) %>: <%= @meeting.participants.attended.sort.join("; ") %></li>
diff --git a/modules/meeting/app/views/meeting_mailer/icalendar_notification.text.erb b/modules/meeting/app/views/meeting_mailer/icalendar_notification.text.erb
index 89b8ed4fef39..077cc974d317 100644
--- a/modules/meeting/app/views/meeting_mailer/icalendar_notification.text.erb
+++ b/modules/meeting/app/views/meeting_mailer/icalendar_notification.text.erb
@@ -3,7 +3,7 @@
 
 <%= t(:text_notificiation_invited) %>
 
-<%=t :label_meeting_date_time %>: <%= format_time_as_date @meeting.start_time %> <%= format_time @meeting.start_time, false %>-<%= format_time @meeting.end_time, false %> <%= @formatted_timezone %>
+<%=t :label_meeting_date_time %>: <%= format_time_as_date @meeting.start_time %> <%= format_time @meeting.start_time, include_date: false %>-<%= format_time @meeting.end_time, include_date: false %> (<%= formatted_time_zone_offset %>)
 <%=Meeting.human_attribute_name(:location) %>: <%= @meeting.location %>
 <%=Meeting.human_attribute_name(:participants_invited) %>: <%= @meeting.participants.invited.sort.join("; ") %>
 <%=Meeting.human_attribute_name(:participants_attended) %>: <%= @meeting.participants.attended.sort.join("; ") %>
diff --git a/modules/meeting/app/views/meeting_mailer/invited.html.erb b/modules/meeting/app/views/meeting_mailer/invited.html.erb
index a44634c827eb..499075a3b736 100644
--- a/modules/meeting/app/views/meeting_mailer/invited.html.erb
+++ b/modules/meeting/app/views/meeting_mailer/invited.html.erb
@@ -45,9 +45,9 @@ See COPYRIGHT and LICENSE files for more details.
               <%= I18n.t(:label_meeting_date_time) %>
             </td>
             <td style="<%= placeholder_text_styles %>">
-              <%= format_time_as_date @meeting.start_time %> <%= format_time @meeting.start_time, false %>
+              <%= format_time_as_date @meeting.start_time %> <%= format_time @meeting.start_time, include_date: false %>
               -
-              <%= format_time @meeting.end_time, false %> <%= Time.zone %>
+              <%= format_time @meeting.end_time, include_date: false %> (<%= formatted_time_zone_offset %>)
             </td>
           </tr>
           <% if @meeting.location.present? %>
diff --git a/modules/meeting/app/views/meeting_mailer/invited.text.erb b/modules/meeting/app/views/meeting_mailer/invited.text.erb
index 3c4160486f58..36f7ce7116a3 100644
--- a/modules/meeting/app/views/meeting_mailer/invited.text.erb
+++ b/modules/meeting/app/views/meeting_mailer/invited.text.erb
@@ -32,7 +32,7 @@ See COPYRIGHT and LICENSE files for more details.
 <%= @meeting.project.name %>: <%= @meeting.title %> (<%= meeting_url(@meeting) %>)
 <%= @meeting.author %>
 
-<%=t :label_meeting_date_time %>: <%= format_time_as_date @meeting.start_time %> <%= format_time @meeting.start_time, false %>-<%= format_time @meeting.end_time, false %> <%= Time.zone %>
+<%=t :label_meeting_date_time %>: <%= format_time_as_date @meeting.start_time %> <%= format_time @meeting.start_time, include_date: false %>-<%= format_time @meeting.end_time, include_date: false %> (<%= formatted_time_zone_offset %>)
 <%= Meeting.human_attribute_name(:location) %>: <%= @meeting.location %>
 <%= Meeting.human_attribute_name(:participants_invited) %>: <%= @meeting.participants.invited.sort.join("; ") %>
 <%= Meeting.human_attribute_name(:participants_attended) %>: <%= @meeting.participants.attended.sort.join("; ") %>
diff --git a/modules/meeting/app/views/meeting_mailer/rescheduled.html.erb b/modules/meeting/app/views/meeting_mailer/rescheduled.html.erb
index 29744c72316d..b0cd462ebbb5 100644
--- a/modules/meeting/app/views/meeting_mailer/rescheduled.html.erb
+++ b/modules/meeting/app/views/meeting_mailer/rescheduled.html.erb
@@ -46,9 +46,9 @@ See COPYRIGHT and LICENSE files for more details.
             </td>
             <td style="<%= placeholder_text_styles %>">
               <s>
-                <%= format_time_as_date @changes[:old_start] %> <%= format_time @changes[:old_start], false %>
+                <%= format_time_as_date @changes[:old_start] %> <%= format_time @changes[:old_start], include_date: false %>
                 -
-                <%= format_time (@changes[:old_start] + @changes[:old_duration].hours), false %> <%= Time.zone %>
+                <%= format_time (@changes[:old_start] + @changes[:old_duration].hours), include_date: false %> (<%= formatted_time_zone_offset %>)
               </s>
             </td>
           </tr>
@@ -57,9 +57,9 @@ See COPYRIGHT and LICENSE files for more details.
               <%= t('meeting.email.rescheduled.new_date_time') %>
             </td>
             <td style="<%= placeholder_text_styles('font-weight': 'bold') %>">
-              <%= format_time_as_date @changes[:new_start] %> <%= format_time @changes[:new_start], false %>
+              <%= format_time_as_date @changes[:new_start] %> <%= format_time @changes[:new_start], include_date: false %>
               -
-              <%= format_time (@changes[:new_start] + @changes[:new_duration].hours), false %> <%= Time.zone %>
+              <%= format_time (@changes[:new_start] + @changes[:new_duration].hours), include_date: false %> (<%= formatted_time_zone_offset %>)
             </td>
           </tr>
           <% if @meeting.location.present? %>
diff --git a/modules/meeting/app/views/meeting_mailer/rescheduled.text.erb b/modules/meeting/app/views/meeting_mailer/rescheduled.text.erb
index dc4e8250bee8..788101237038 100644
--- a/modules/meeting/app/views/meeting_mailer/rescheduled.text.erb
+++ b/modules/meeting/app/views/meeting_mailer/rescheduled.text.erb
@@ -35,7 +35,7 @@ See COPYRIGHT and LICENSE files for more details.
       title: @meeting.title) %>
 
 <%= t('meeting.email.rescheduled.old_date_time') %>:
-<%= format_time_as_date @changes[:old_start] %> <%= format_time @changes[:old_start], false %> - <%= format_time (@changes[:old_start] + @changes[:old_duration]), false %> <%= Time.zone %>
+<%= format_time_as_date @changes[:old_start] %> <%= format_time @changes[:old_start], include_date: false %> - <%= format_time (@changes[:old_start] + @changes[:old_duration]), include_date: false %> (<%= formatted_time_zone_offset %>}
 
 <%= t('meeting.email.rescheduled.new_date_time') %>:
-<%= format_time_as_date @changes[:new_start] %> <%= format_time @changes[:new_start], false %> - <%= format_time (@changes[:new_start] + @changes[:new_duration]), false %> <%= Time.zone %>
+<%= format_time_as_date @changes[:new_start] %> <%= format_time @changes[:new_start], include_date: false %> - <%= format_time (@changes[:new_start] + @changes[:new_duration]), include_date: false %> (<%= formatted_time_zone_offset %>}
diff --git a/modules/meeting/app/views/meetings/_form.html.erb b/modules/meeting/app/views/meetings/_form.html.erb
index af48e62e620b..abf981a7d722 100644
--- a/modules/meeting/app/views/meetings/_form.html.erb
+++ b/modules/meeting/app/views/meetings/_form.html.erb
@@ -160,7 +160,7 @@ See COPYRIGHT and LICENSE files for more details.
         <%= Meeting.human_attribute_name(:start_time) %>
         <label lang="en">
           <%= t(:label_time_zone) %>
-          <%= Time.zone.to_s %>
+          <%= formatted_time_zone_offset %>
         </label>
       </label>
       <%= f.text_field :start_time_hour,
@@ -169,7 +169,7 @@ See COPYRIGHT and LICENSE files for more details.
                        type: 'time',
                        no_label: true,
                        step: 5.minutes,
-                       suffix: Time.zone.to_s,
+                       suffix: formatted_time_zone_offset,
                        container_class: '-xslim' %>
     </div>
   </div>
diff --git a/modules/meeting/app/views/meetings/show.html.erb b/modules/meeting/app/views/meetings/show.html.erb
index 15b97a0bd21d..ecb297e3b64e 100644
--- a/modules/meeting/app/views/meetings/show.html.erb
+++ b/modules/meeting/app/views/meetings/show.html.erb
@@ -100,8 +100,8 @@ See COPYRIGHT and LICENSE files for more details.
     </div>
     <div class="grid-content small-6">
       <p>
-        <strong><%= Meeting.human_attribute_name(:start_time) %></strong>: <%= format_date @meeting.start_time %> <%= format_time @meeting.start_time, false %>
-        - <%= format_time @meeting.end_time, false %> <%= Time.zone %></p>
+        <strong><%= Meeting.human_attribute_name(:start_time) %></strong>: <%= format_date @meeting.start_time %> <%= format_time @meeting.start_time, include_date: false %>
+        - <%= format_time @meeting.end_time, include_date: false %> <%= formatted_time_zone_offset %></p>
     </div>
     <div class="grid-content small-6">
       <p>
diff --git a/modules/meeting/spec/features/meetings_copy_spec.rb b/modules/meeting/spec/features/meetings_copy_spec.rb
index 9218095fda73..3a81be1d576a 100644
--- a/modules/meeting/spec/features/meetings_copy_spec.rb
+++ b/modules/meeting/spec/features/meetings_copy_spec.rb
@@ -34,7 +34,7 @@
   shared_let(:user) do
     create(:user,
            member_with_permissions: { project => permissions }).tap do |u|
-      u.pref[:time_zone] = "UTC"
+      u.pref[:time_zone] = "Etc/UTC"
 
       u.save!
     end
@@ -66,7 +66,7 @@
     start_of_meeting = start_time.strftime(twelve_hour_format)
     end_of_meeting = (start_time + meeting.duration.hours).strftime(twelve_hour_format)
 
-    "Start time: #{date} #{start_of_meeting} - #{end_of_meeting} (GMT+00:00) UTC"
+    "Start time: #{date} #{start_of_meeting} - #{end_of_meeting} UTC+00:00"
   end
 
   before do
diff --git a/modules/meeting/spec/features/meetings_new_spec.rb b/modules/meeting/spec/features/meetings_new_spec.rb
index 29dcf6148c42..fa7bc94c48fb 100644
--- a/modules/meeting/spec/features/meetings_new_spec.rb
+++ b/modules/meeting/spec/features/meetings_new_spec.rb
@@ -33,7 +33,7 @@
 RSpec.describe "Meetings new", :js, with_cuprite: false do
   shared_let(:project) { create(:project, enabled_module_names: %w[meetings]) }
   shared_let(:admin) { create(:admin) }
-  let(:time_zone) { "utc" }
+  let(:time_zone) { "Etc/UTC" }
   let(:user) do
     create(:user,
            lastname: "First",
diff --git a/modules/meeting/spec/features/structured_meetings/mobile_structure_meeting_spec.rb b/modules/meeting/spec/features/structured_meetings/mobile_structure_meeting_spec.rb
index 249744c00efb..eefba3dd0817 100644
--- a/modules/meeting/spec/features/structured_meetings/mobile_structure_meeting_spec.rb
+++ b/modules/meeting/spec/features/structured_meetings/mobile_structure_meeting_spec.rb
@@ -41,7 +41,7 @@
            lastname: "First",
            member_with_permissions: { project => %i[view_meetings create_meetings edit_meetings delete_meetings manage_agendas
                                                     close_meeting_agendas view_work_packages] }).tap do |u|
-      u.pref[:time_zone] = "utc"
+      u.pref[:time_zone] = "Etc/UTC"
 
       u.save!
     end
diff --git a/modules/meeting/spec/features/structured_meetings/structured_meeting_crud_spec.rb b/modules/meeting/spec/features/structured_meetings/structured_meeting_crud_spec.rb
index a2800ec121ba..c8f92c517370 100644
--- a/modules/meeting/spec/features/structured_meetings/structured_meeting_crud_spec.rb
+++ b/modules/meeting/spec/features/structured_meetings/structured_meeting_crud_spec.rb
@@ -42,7 +42,7 @@
            lastname: "First",
            member_with_permissions: { project => %i[view_meetings create_meetings edit_meetings delete_meetings manage_agendas
                                                     view_work_packages] }).tap do |u|
-      u.pref[:time_zone] = "utc"
+      u.pref[:time_zone] = "Etc/UTC"
 
       u.save!
     end
diff --git a/modules/meeting/spec/features/structured_meetings/structured_meeting_participant_spec.rb b/modules/meeting/spec/features/structured_meetings/structured_meeting_participant_spec.rb
index f2a2816d897e..e89f4494ef90 100644
--- a/modules/meeting/spec/features/structured_meetings/structured_meeting_participant_spec.rb
+++ b/modules/meeting/spec/features/structured_meetings/structured_meeting_participant_spec.rb
@@ -41,7 +41,7 @@
            lastname: "First",
            member_with_permissions: { project => %i[view_meetings create_meetings edit_meetings delete_meetings manage_agendas
                                                     close_meeting_agendas view_work_packages] }).tap do |u|
-      u.pref[:time_zone] = "utc"
+      u.pref[:time_zone] = "Etc/UTC"
 
       u.save!
     end
diff --git a/modules/meeting/spec/features/structured_meetings/turbo_links_spec.rb b/modules/meeting/spec/features/structured_meetings/turbo_links_spec.rb
index 1cb1af730276..7eb01c1d3c9f 100644
--- a/modules/meeting/spec/features/structured_meetings/turbo_links_spec.rb
+++ b/modules/meeting/spec/features/structured_meetings/turbo_links_spec.rb
@@ -42,7 +42,7 @@
            lastname: "First",
            member_with_permissions: { project => %i[view_meetings create_meetings edit_meetings delete_meetings manage_agendas
                                                     view_work_packages] }).tap do |u|
-      u.pref[:time_zone] = "utc"
+      u.pref[:time_zone] = "Etc/UTC"
 
       u.save!
     end
diff --git a/modules/meeting/spec/mailers/meeting_mailer_spec.rb b/modules/meeting/spec/mailers/meeting_mailer_spec.rb
index 8d6eb3d32281..b5fc84a3747a 100644
--- a/modules/meeting/spec/mailers/meeting_mailer_spec.rb
+++ b/modules/meeting/spec/mailers/meeting_mailer_spec.rb
@@ -47,6 +47,8 @@
   let(:meeting_agenda) do
     create(:meeting_agenda, meeting:)
   end
+  let(:tokyo_offset) { "UTC#{ActiveSupport::TimeZone['Asia/Tokyo'].now.formatted_offset}" }
+  let(:berlin_offset) { "UTC#{ActiveSupport::TimeZone['Europe/Berlin'].now.formatted_offset}" }
 
   before do
     User.current = author
@@ -88,11 +90,9 @@
     context "with a recipient with another time zone" do
       let!(:preference) { watcher1.pref.update(time_zone: "Asia/Tokyo") }
 
-      it "renders the mail with the correcet locale" do
-        expect(mail.text_part.body).to include("Tokyo")
-        expect(mail.text_part.body).to include("GMT+09:00")
-        expect(mail.html_part.body).to include("Tokyo")
-        expect(mail.html_part.body).to include("GMT+09:00")
+      it "renders the mail with the correct locale" do
+        expect(mail.text_part.body).to include(tokyo_offset)
+        expect(mail.html_part.body).to include(tokyo_offset)
 
         expect(mail.to).to contain_exactly(watcher1.mail)
       end
@@ -111,8 +111,8 @@
 
         it "renders the mail with the correct locale" do
           expect(mail.html_part.body).to include("11/09/2021 11:00 PM")
-          expect(mail.html_part.body).to include("12:00 AM (GMT+01:00) Europe/Berlin")
-          expect(mail.text_part.body).to include("11/09/2021 11:00 PM-12:00 AM (GMT+01:00) Europe/Berlin")
+          expect(mail.html_part.body).to include("12:00 AM (#{berlin_offset})")
+          expect(mail.text_part.body).to include("11/09/2021 11:00 PM-12:00 AM (#{berlin_offset})")
 
           expect(mail.to).to contain_exactly(author.mail)
         end
@@ -124,9 +124,9 @@
 
         it "renders the mail with the correct locale" do
           expect(mail.html_part.body).to include("11/10/2021 07:00 AM")
-          expect(mail.html_part.body).to include("08:00 AM (GMT+09:00) Asia/Tokyo")
+          expect(mail.html_part.body).to include("08:00 AM (#{tokyo_offset})")
 
-          expect(mail.text_part.body).to include("11/10/2021 07:00 AM-08:00 AM (GMT+09:00) Asia/Tokyo")
+          expect(mail.text_part.body).to include("11/10/2021 07:00 AM-08:00 AM (#{tokyo_offset})")
 
           expect(mail.to).to contain_exactly(watcher1.mail)
         end
@@ -160,7 +160,7 @@
         expect(body).to include(meeting.project.name)
         expect(body).to include(meeting.title)
         expect(body).to include(meeting.location)
-        expect(body).to include("01/19/2021 11:00 AM-12:00 PM (GMT+01:00) Europe/Berlin")
+        expect(body).to include("01/19/2021 11:00 AM-12:00 PM (#{berlin_offset})")
         expect(body).to include(meeting.participants[0].name)
         expect(body).to include(meeting.participants[1].name)
       end
@@ -174,7 +174,7 @@
         expect(body).to include(meeting.title)
         expect(body).to include(meeting.location)
         expect(body).to include("01/19/2021 11:00 AM")
-        expect(body).to include("12:00 PM (GMT+01:00) Europe/Berlin")
+        expect(body).to include("12:00 PM (#{berlin_offset})")
         expect(body).to include(meeting.participants[0].name)
         expect(body).to include(meeting.participants[1].name)
       end
@@ -207,9 +207,9 @@
       let(:mail) { described_class.icalendar_notification(meeting, watcher1, author) }
 
       it "renders the mail with the correct locale" do
-        expect(mail.text_part.body).to include("01/19/2021 07:00 PM-08:00 PM (GMT+09:00) Asia/Tokyo")
+        expect(mail.text_part.body).to include("01/19/2021 07:00 PM-08:00 PM (#{tokyo_offset})")
         expect(mail.html_part.body).to include("01/19/2021 07:00 PM")
-        expect(mail.html_part.body).to include("08:00 PM (GMT+09:00) Asia/Tokyo")
+        expect(mail.html_part.body).to include("08:00 PM (#{tokyo_offset})")
 
         expect(mail.to).to contain_exactly(watcher1.mail)
       end
@@ -227,9 +227,9 @@
         let(:mail) { described_class.icalendar_notification(meeting, author, author) }
 
         it "renders the mail with the correct locale" do
-          expect(mail.text_part.body).to include("11/09/2021 11:00 PM-12:00 AM (GMT+01:00) Europe/Berlin")
+          expect(mail.text_part.body).to include("11/09/2021 11:00 PM-12:00 AM (#{berlin_offset})")
           expect(mail.html_part.body).to include("11/09/2021 11:00 PM")
-          expect(mail.html_part.body).to include("12:00 AM (GMT+01:00) Europe/Berlin")
+          expect(mail.html_part.body).to include("12:00 AM (#{berlin_offset})")
 
           expect(mail.to).to contain_exactly(author.mail)
         end
@@ -240,8 +240,8 @@
         let!(:preference) { watcher1.pref.update(time_zone: "Asia/Tokyo") }
 
         it "renders the mail with the correct locale" do
-          expect(mail.text_part.body).to include("11/10/2021 07:00 AM-08:00 AM (GMT+09:00) Asia/Tokyo")
-          expect(mail.html_part.body).to include("11/10/2021 07:00 AM-08:00 AM (GMT+09:00) Asia/Tokyo")
+          expect(mail.text_part.body).to include("11/10/2021 07:00 AM-08:00 AM (#{tokyo_offset})")
+          expect(mail.html_part.body).to include("11/10/2021 07:00 AM-08:00 AM (#{tokyo_offset})")
 
           expect(mail.to).to contain_exactly(watcher1.mail)
         end
@@ -253,8 +253,9 @@ def check_meeting_mail_content(body)
     expect(body).to include(meeting.project.name)
     expect(body).to include(meeting.title)
     expect(body).to include(i18n.format_date(meeting.start_date))
-    expect(body).to include(i18n.format_time(meeting.start_time, false))
-    expect(body).to include(i18n.format_time(meeting.end_time, false))
+    expect(body).to include(i18n.format_time(meeting.start_time, include_date: false))
+    expect(body).to include(i18n.format_time(meeting.end_time, include_date: false))
+    expect(body).to include(i18n.formatted_time_zone_offset)
     expect(body).to include(meeting.participants[0].name)
     expect(body).to include(meeting.participants[1].name)
   end
diff --git a/modules/xls_export/app/models/xls_export/concerns/spreadsheet_builder.rb b/modules/xls_export/app/models/xls_export/concerns/spreadsheet_builder.rb
index 60780f76ddb1..1c2cc22ccb07 100644
--- a/modules/xls_export/app/models/xls_export/concerns/spreadsheet_builder.rb
+++ b/modules/xls_export/app/models/xls_export/concerns/spreadsheet_builder.rb
@@ -79,7 +79,7 @@ def headers
       def xls_export_filename
         sane_filename(
           "#{Setting.app_title} #{spreadsheet_title} \
-          #{format_time_as_date(Time.zone.now, '%Y-%m-%d')}.xls"
+          #{format_time_as_date(Time.zone.now, format: '%Y-%m-%d')}.xls"
         )
       end
     end
diff --git a/spec/lib/api/v3/user_preferences/user_preference_representer_rendering_spec.rb b/spec/lib/api/v3/user_preferences/user_preference_representer_rendering_spec.rb
index 8aa1e05291ea..0c23e3199c3e 100644
--- a/spec/lib/api/v3/user_preferences/user_preference_representer_rendering_spec.rb
+++ b/spec/lib/api/v3/user_preferences/user_preference_representer_rendering_spec.rb
@@ -62,8 +62,8 @@
     context "without a timezone set" do
       let(:preference) { build(:user_preference, time_zone: "") }
 
-      it "shows the timeZone as nil" do
-        expect(subject).to be_json_eql(nil.to_json).at_path("timeZone")
+      it "shows the timeZone as utc" do
+        expect(subject).to be_json_eql("Etc/UTC".to_json).at_path("timeZone")
       end
     end
 
diff --git a/spec/lib/redmine/i18n_spec.rb b/spec/lib/redmine/i18n_spec.rb
index 34585e021e7f..9512c0234ba7 100644
--- a/spec/lib/redmine/i18n_spec.rb
+++ b/spec/lib/redmine/i18n_spec.rb
@@ -35,43 +35,35 @@ module OpenProject
     let(:format) { "%d/%m/%Y" }
     let(:user) { build_stubbed(:user) }
 
-    after do
-      Time.zone = nil
-    end
+    describe "#format_time_as_date" do
+      current_user { build_stubbed(:user, preferences: { time_zone: user_time_zone }) }
 
-    describe "with user time zone" do
-      before do
-        login_as user
-        allow(user).to receive(:time_zone).and_return(ActiveSupport::TimeZone["Athens"])
-      end
+      describe "with user time zone" do
+        let(:user_time_zone) { "Europe/Athens" }
 
-      it "returns a date in the user timezone for a utc timestamp" do
-        Time.zone = "UTC"
-        time = Time.zone.local(2013, 6, 30, 23, 59)
-        expect(format_time_as_date(time, format)).to eq "01/07/2013"
-      end
+        it "returns a date string in the user timezone for a utc timestamp" do
+          time = ActiveSupport::TimeZone["UTC"].local(2013, 6, 30, 23, 59)
+          expect(format_time_as_date(time, format:)).to eq "01/07/2013"
+        end
 
-      it "returns a date in the user timezone for a non-utc timestamp" do
-        Time.zone = "Berlin"
-        time = Time.zone.local(2013, 6, 30, 23, 59)
-        expect(format_time_as_date(time, format)).to eq "01/07/2013"
+        it "returns a date string in the user timezone for a non-utc timestamp" do
+          time = ActiveSupport::TimeZone["Berlin"].local(2013, 6, 30, 23, 59)
+          expect(format_time_as_date(time, format:)).to eq "01/07/2013"
+        end
       end
-    end
 
-    describe "without user time zone" do
-      before { allow(User.current).to receive(:time_zone).and_return(nil) }
+      describe "without user time zone" do
+        let(:user_time_zone) { "" }
 
-      it "returns a date in the local system timezone for a utc timestamp" do
-        Time.zone = "UTC"
-        time = Time.zone.local(2013, 6, 30, 23, 59)
-        allow(time).to receive(:localtime).and_return(ActiveSupport::TimeZone["Athens"].local(2013, 7, 1, 1, 59))
-        expect(format_time_as_date(time, format)).to eq "01/07/2013"
-      end
+        it "returns a date string in the utc timezone for a utc timestamp" do
+          time = ActiveSupport::TimeZone["UTC"].local(2013, 6, 30, 23, 59)
+          expect(format_time_as_date(time, format:)).to eq "30/06/2013"
+        end
 
-      it "returns a date in the original timezone for a non-utc timestamp" do
-        Time.zone = "Berlin"
-        time = Time.zone.local(2013, 6, 30, 23, 59)
-        expect(format_time_as_date(time, format)).to eq "30/06/2013"
+        it "returns a date string in the utc timezone for a non-utc timestamp" do
+          time = ActiveSupport::TimeZone["Berlin"].local(2013, 6, 30, 23, 59)
+          expect(format_time_as_date(time, format:)).to eq "30/06/2013"
+        end
       end
     end
 
@@ -271,7 +263,10 @@ module OpenProject
       time_format: "%H %M",
       date_format: "%d %m %Y"
     } do
-      let!(:now) { Time.parse("2011-02-20 15:45:22") }
+      let(:user_time_zone) { "" }
+      let(:now) { Time.zone.parse("2011-02-20 15:45:22") }
+
+      current_user { build_stubbed(:user, preferences: { time_zone: user_time_zone }) }
 
       it "with date and hours" do
         expect(format_time(now))
@@ -279,18 +274,33 @@ module OpenProject
       end
 
       it "with only hours" do
-        expect(format_time(now, false))
+        expect(format_time(now, include_date: false))
           .to eql now.strftime("%H %M")
       end
 
-      it "with a utc to date and hours" do
-        expect(format_time(now.utc))
-          .to eql now.localtime.strftime("%d %m %Y %H %M")
+      it "renders correctly for only hours and when providing a custom format" do
+        expect(format_time(now, include_date: false, format: "%H:%M"))
+          .to eql now.strftime("%H:%M")
       end
 
-      it "with a utce to only hours" do
-        expect(format_time(now.utc, false))
-          .to eql now.localtime.strftime("%H %M")
+      context "with another time zone configured for the user" do
+        # Kathmandu has a +05:45 offset
+        let(:user_time_zone) { "Kathmandu" }
+
+        it "renders correctly for data and hours" do
+          expect(format_time(now))
+            .to eql "20 02 2011 21 30"
+        end
+
+        it "renders correctly for only hours" do
+          expect(format_time(now, include_date: false))
+            .to eql "21 30"
+        end
+
+        it "renders correctly for only hours and when providing a custom format" do
+          expect(format_time(now, include_date: false, format: "%H:%M"))
+            .to eql "21:30"
+        end
       end
 
       context "with a different format defined", with_settings: {
@@ -303,7 +313,7 @@ module OpenProject
         end
 
         it "renders only hours" do
-          expect(format_time(now, false))
+          expect(format_time(now, include_date: false))
             .to eql "15:45"
         end
       end
@@ -318,7 +328,7 @@ module OpenProject
         end
 
         it "falls back to default for only hours" do
-          expect(format_time(now, false))
+          expect(format_time(now, include_date: false))
             .to eql "03:45 PM"
         end
 
@@ -333,7 +343,7 @@ module OpenProject
 
             it "raises no error for only hours" do
               described_class.with_locale lang do
-                expect { format_time(now, false) }
+                expect { format_time(now, include_date: false) }
                   .not_to raise_error
               end
             end
@@ -351,7 +361,7 @@ module OpenProject
         end
 
         it "falls back to default for only hours" do
-          expect(format_time(now, false))
+          expect(format_time(now, include_date: false))
             .to eql "03:45 PM"
         end
       end
@@ -366,12 +376,53 @@ module OpenProject
         end
 
         it "falls back to default for only hours" do
-          expect(format_time(now, false))
+          expect(format_time(now, include_date: false))
             .to eql "15:45"
         end
       end
     end
 
+    describe "#formatted_time_zone_offset" do
+      current_user { build_stubbed(:user, preferences: { time_zone: user_time_zone }) }
+      let(:user_time_zone) { "" }
+
+      let(:berlin_gmt) { ActiveSupport::TimeZone["Europe/Berlin"].now.utc_offset == 7200 ? "UTC+02:00" : "UTC+01:00" }
+
+      context "with the current user having set a time zone" do
+        let(:user_time_zone) { "Europe/Berlin" }
+
+        it "renders the time zone of the user" do
+          expect(formatted_time_zone_offset).to eql berlin_gmt
+        end
+      end
+
+      context "without the current user having a time zone and no default one configured" do
+        let(:user_time_zone) { "" }
+
+        it "renders the UTC time zone" do
+          expect(formatted_time_zone_offset).to eql "UTC+00:00"
+        end
+      end
+
+      context "without the current user having a time zone but with a default one configured",
+              with_settings: { user_default_timezone: "Europe/Berlin" } do
+        let(:user_time_zone) { "" }
+
+        it "renders the default time zone" do
+          expect(formatted_time_zone_offset).to eql berlin_gmt
+        end
+      end
+
+      context "without the current user having a time zone and also a default one configured",
+              with_settings: { user_default_timezone: "America/Atlanta" } do
+        let(:user_time_zone) { "Europe/Berlin" }
+
+        it "renders the default time zone" do
+          expect(formatted_time_zone_offset).to eql berlin_gmt
+        end
+      end
+    end
+
     describe "day names" do
       valid_languages.each do |lang|
         context "for locale #{lang}" do
diff --git a/spec/models/deleted_user_spec.rb b/spec/models/deleted_user_spec.rb
index 03193804152d..5b477505d99d 100644
--- a/spec/models/deleted_user_spec.rb
+++ b/spec/models/deleted_user_spec.rb
@@ -48,7 +48,7 @@
   end
 
   describe "#time_zone" do
-    it { expect(user.time_zone).to be_nil }
+    it { expect(user.time_zone).to eql ActiveSupport::TimeZone["Etc/UTC"] }
   end
 
   describe "#rss_key" do
diff --git a/spec/models/user_preference_spec.rb b/spec/models/user_preference_spec.rb
index b0cd3fdff46d..b482481ae070 100644
--- a/spec/models/user_preference_spec.rb
+++ b/spec/models/user_preference_spec.rb
@@ -222,4 +222,48 @@
       expect(subject[:auto_hide_popups]).to eql(value_auto_hide_popups)
     end
   end
+
+  describe "#time_zone" do
+    context "with a time zone set and a default configured", with_settings: { user_default_timezone: "America/Los_Angeles" } do
+      let(:settings) { { "time_zone" => "Africa/Algiers" } }
+
+      it "returns the time zone set" do
+        expect(preference.time_zone).to eql "Africa/Algiers"
+      end
+    end
+
+    context "with no time zone configured but a default", with_settings: { user_default_timezone: "America/Los_Angeles" } do
+      it "returns the default time zone" do
+        expect(preference.time_zone).to eql "America/Los_Angeles"
+      end
+    end
+
+    context "with neiter a time zone configured nor a default one", with_settings: { user_default_timezone: "" } do
+      it "returns UTC" do
+        expect(preference.time_zone).to eql "Etc/UTC"
+      end
+    end
+  end
+
+  describe "#time_zone?" do
+    context "with a time zone set and a default configured", with_settings: { user_default_timezone: "America/Los_Angeles" } do
+      let(:settings) { { "time_zone" => "Africa/Algiers" } }
+
+      it "is true" do
+        expect(preference).to be_time_zone
+      end
+    end
+
+    context "with no time zone configured but a default", with_settings: { user_default_timezone: "America/Los_Angeles" } do
+      it "is false" do
+        expect(preference).not_to be_time_zone
+      end
+    end
+
+    context "with neiter a time zone configured nor a default one", with_settings: { user_default_timezone: "" } do
+      it "is false" do
+        expect(preference).not_to be_time_zone
+      end
+    end
+  end
 end
diff --git a/spec/models/user_spec.rb b/spec/models/user_spec.rb
index 2cb717f4e12c..66d7db19d217 100644
--- a/spec/models/user_spec.rb
+++ b/spec/models/user_spec.rb
@@ -823,6 +823,39 @@ def build_user_double_with_expired_password(is_expired)
     end
   end
 
+  describe "#time_zone" do
+    let(:user) { build(:user, preferences:) }
+
+    context "with an existing time zone in the prefs" do
+      let(:preferences) { { "time_zone" => "Europe/Athens" } }
+
+      it "returns the matching ActiveSupport::TimeZone" do
+        expect(user.time_zone)
+          .to eql ActiveSupport::TimeZone["Europe/Athens"]
+      end
+    end
+
+    context "with an invalid time zone" do
+      # Would need to be Etc/UTC or UTC to be valid
+      let(:preferences) { { "time_zone" => "utc" } }
+
+      it "returns the utc ActiveSupport::TimeZone" do
+        expect(user.time_zone)
+          .to eql ActiveSupport::TimeZone["Etc/UTC"]
+      end
+    end
+
+    context "without a time zone" do
+      # Would need to be Etc/UTC or UTC to be valid
+      let(:preferences) { {} }
+
+      it "returns the utc ActiveSupport::TimeZone" do
+        expect(user.time_zone)
+          .to eql ActiveSupport::TimeZone["Etc/UTC"]
+      end
+    end
+  end
+
   describe "#find_by_mail" do
     let!(:user1) { create(:user, mail: "foo+test@example.org") }
     let!(:user2) { create(:user, mail: "foo@example.org") }
diff --git a/spec/models/users/default_timezone_spec.rb b/spec/models/users/default_timezone_spec.rb
index e0a83088ce78..f0f2b6e5cf61 100644
--- a/spec/models/users/default_timezone_spec.rb
+++ b/spec/models/users/default_timezone_spec.rb
@@ -32,8 +32,8 @@
   let(:user) { create(:user) }
 
   context "with no system default set" do
-    it "is not set" do
-      expect(user.pref.time_zone).to be_nil
+    it "is still set to Etc/UTC as that will be calculated with internally" do
+      expect(user.pref.time_zone).to eq "Etc/UTC"
     end
   end
 
diff --git a/spec/models/work_packages/pdf_export/work_package_list_to_pdf_gantt_spec.rb b/spec/models/work_packages/pdf_export/work_package_list_to_pdf_gantt_spec.rb
index a3574502c794..054584134b1d 100644
--- a/spec/models/work_packages/pdf_export/work_package_list_to_pdf_gantt_spec.rb
+++ b/spec/models/work_packages/pdf_export/work_package_list_to_pdf_gantt_spec.rb
@@ -78,7 +78,7 @@ def respond_to?(*)
     end
   end
   let(:export_time) { DateTime.new(2024, 4, 22, 12, 37) }
-  let(:export_time_formatted) { format_time(export_time, true) }
+  let(:export_time_formatted) { format_time(export_time, include_date: true) }
   let(:export) do
     login_as(user)
     work_packages
diff --git a/spec/models/work_packages/pdf_export/work_package_list_to_pdf_spec.rb b/spec/models/work_packages/pdf_export/work_package_list_to_pdf_spec.rb
index 9b8443df9876..6bcce108994b 100644
--- a/spec/models/work_packages/pdf_export/work_package_list_to_pdf_spec.rb
+++ b/spec/models/work_packages/pdf_export/work_package_list_to_pdf_spec.rb
@@ -62,7 +62,7 @@
            member_with_permissions: { project => %w[view_work_packages export_work_packages] })
   end
   let(:export_time) { DateTime.new(2023, 6, 30, 23, 59) }
-  let(:export_time_formatted) { format_time(export_time, true) }
+  let(:export_time_formatted) { format_time(export_time, include_date: true) }
   let(:work_package_parent) do
     create(:work_package,
            project:,
diff --git a/spec/models/work_packages/pdf_export/work_package_to_pdf_spec.rb b/spec/models/work_packages/pdf_export/work_package_to_pdf_spec.rb
index f898900a00eb..fbd4a11b2ffb 100644
--- a/spec/models/work_packages/pdf_export/work_package_to_pdf_spec.rb
+++ b/spec/models/work_packages/pdf_export/work_package_to_pdf_spec.rb
@@ -91,7 +91,7 @@
   let(:category) { create(:category, project:, name: "Demo") }
   let(:version) { create(:version, project:) }
   let(:export_time) { DateTime.new(2023, 6, 30, 23, 59) }
-  let(:export_time_formatted) { format_time(export_time, true) }
+  let(:export_time_formatted) { format_time(export_time, include_date: true) }
   let(:image_path) { Rails.root.join("spec/fixtures/files/image.png") }
   let(:priority) { create(:priority_normal) }
   let(:image_attachment) { Attachment.new author: user, file: File.open(image_path) }