Add DTSTAMP to events to help updates flow through to clients

DTSTAMP is supposed to be in UTC, so I needed to add a .rb plugin. I
hope it works on GH Pages.

_layouts/ical.ics

@@ -5,12 +5,17 @@ VERSION:2.0
 PRODID:-//{{ site.domain }}//{{ site.title }}//EN
 CALSCALE:GREGORIAN{% assign events_data = site.pages | where: "layout", "event" | sort: "path" | reverse %}{% for event in events_data %}
 BEGIN:VEVENT
-SUMMARY:{{ event.title }}
 URL:{{ event.url | prepend: site.url }}
 UID:{{ event.url | prepend: site.domain }}
+{%- comment -%}
+DTSTAMP is when this file was generated, LAST-MOD is when the content of this event changed and SEQUENCE is an int that increments when the event changes substantially (such that RSVPs should be reconsidered).
+{% endcomment %}
+DTSTAMP: {{ site.time | to_utc | date: "%Y%m%dT%H%M%SZ" }}
+CLASS: PUBLIC
+SUMMARY:{{ event.title }} and dey say
 ORGANIZER;CN="{{ site.title }}":MAILTO:{{ site.email }}
-DTSTART;TZID=/{{ site.timezone }}:{{ event.start_time | date: "%Y%m%dT%H%M00" }}{% if event.end_time %}
-DTEND;TZID=/{{ site.timezone }}:{{ event.end_time | date: "%Y%m%dT%H%M00" }}{% else %}
+DTSTART;TZID=/{{ site.timezone }}:{{ event.start_time | date: "%Y%m%dT%H%M%S" }}{% if event.end_time %}
+DTEND;TZID=/{{ site.timezone }}:{{ event.end_time | date: "%Y%m%dT%H%M%S" }}{% else %}
 DURATION:PT2H{% endif %}
 LOCATION:{{ event.location.name }}{% if event.location.address %}, {{ event.location.address }}{% endif %}{% if event.location.latitude and event.location.longitude %}
 GEO:{{ event.location.latitude }};{{ event.location.longitude }}{% endif %}

_plugins/utc_filter.rb

@@ -0,0 +1,9 @@
+module Jekyll
+  module UTCFilter
+    def to_utc(date)
+      time(date).utc
+    end
+  end
+end
+
+Liquid::Template.register_filter(Jekyll::UTCFilter)