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extended_datetime.c
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extended_datetime.c
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/*
* Wrapper around C runtime functions in <time.h>, to allow larger values for the year part of a date/time
*
* Microsoft C runtime library limits the year value to a maximum of 3000 (on x64 systems), but ASN.1 type
* GeneralizedTime allows larger values.
*
* Licensed under the BSD 3-clause license:
* https://opensource.org/licenses/BSD-3-Clause
*
* Includes modified public domain source code from StackOverflow:
* https://stackoverflow.com/questions/7960318/math-to-convert-seconds-since-1970-into-date-and-vice-versa
*/
#include <stdbool.h>
#include <stdint.h>
#include <time.h>
#include "extended_datetime.h"
static int_fast64_t epoch_days_from_civil(int_fast64_t y, unsigned m, unsigned d)
{
y -= m <= 2;
const int_fast64_t era = (y >= 0 ? y : y-399) / 400;
const unsigned yoe = (unsigned)(y - era * 400); // [0, 399]
const unsigned doy = (153*(m + (m > 2 ? -3 : 9)) + 2)/5 + d-1; // [0, 365]
const unsigned doe = yoe * 365 + yoe/4 - yoe/100 + doy; // [0, 146096]
return era * 146097 + (int_fast64_t)doe - 719468;
}
static void civil_from_epoch_days(int_fast64_t z, struct tm *tm)
{
z += 719468;
const int_fast64_t era = (z >= 0 ? z : z - 146096) / 146097;
const unsigned doe = (unsigned)(z - era * 146097); // [0, 146096]
const unsigned yoe = (doe - doe/1460 + doe/36524 - doe/146096) / 365; // [0, 399]
const int_fast64_t y = (int_fast64_t)(yoe) + era * 400;
const unsigned doy = doe - (365*yoe + yoe/4 - yoe/100); // [0, 365]
const unsigned mp = (5*doy + 2)/153; // [0, 11]
const unsigned d = doy - (153*mp+2)/5 + 1; // [1, 31]
const unsigned m = mp + (mp < 10 ? 3 : -9); // [1, 12]
tm->tm_year = (int)(y + (m <= 2) - 1900);
tm->tm_mon = m - 1;
tm->tm_mday = d;
}
static unsigned weekday_from_epoch_days(gt_time_t z)
{
return (unsigned)(z >= -4 ? (z+4) % 7 : (z+5) % 7 + 6);
}
static gt_time_t gt_epoch_adjustment(struct tm *save_tm)
{
// save tm in case it was obtained from gmtime()/mktime() by the caller
struct tm backup;
if (save_tm)
backup = *save_tm;
time_t time_now = time(NULL);
struct tm *tm = gmtime(&time_now);
gt_time_t epoch_origin_adjust =
epoch_days_from_civil(tm->tm_year + 1900, tm->tm_mon + 1, tm->tm_mday) * 24 * 3600 + tm->tm_hour * 3600 + tm->tm_min * 60 + tm->tm_sec
-
time_now;
if (save_tm)
*save_tm = backup;
return epoch_origin_adjust;
}
static long gt_offset_from_GMT(struct tm arg)
{
time_t specified_time = mktime(&arg);
struct tm gmt_calendar_time = *gmtime(&specified_time);
return specified_time - mktime(&gmt_calendar_time) -
((gmtime(&specified_time)->tm_isdst > 0 ? 1 : 0) - (gmt_calendar_time.tm_isdst > 0 ? 1 : 0)) * 3600;
}
extern gt_time_t gt_timegm(struct tm *tm)
{
gt_time_t year = tm->tm_year + 1900 + tm->tm_mon / 12;
unsigned month = tm->tm_mon % 12 + 1;
gt_time_t
secs_since_epoch = epoch_days_from_civil(year, month, 1) + (tm->tm_mday - 1) * 24 * 3600
+ tm->tm_hour * 3600 + tm->tm_min * 60 + tm->tm_sec,
days_since_epoch = secs_since_epoch;
// Re-construct struct tm
tm->tm_sec = days_since_epoch % 60;
days_since_epoch /= 60;
tm->tm_min = days_since_epoch % 60;
days_since_epoch /= 60;
tm->tm_hour = days_since_epoch % 24;
days_since_epoch /= 24;
civil_from_epoch_days(days_since_epoch, tm);
tm->tm_wday = weekday_from_epoch_days(days_since_epoch);
bool is_leap_year;
if (tm->tm_year % 4 == 0)
if (tm->tm_year % 100 == 0)
if (tm->tm_year % 400 == 0)
is_leap_year = true;
else
is_leap_year = false;
else
is_leap_year = true;
else
is_leap_year = false;
static const unsigned year_days_before_month[12] = { 0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335 };
if (is_leap_year || tm->tm_mon <= 1)
tm->tm_yday = year_days_before_month[12] + tm->tm_mday;
else
tm->tm_yday = year_days_before_month[12] + tm->tm_mday - 1;
tm->tm_yday--; // struct tm counts day-of-year from 0
tm->tm_isdst = 0;
secs_since_epoch -= gt_epoch_adjustment(tm);
return secs_since_epoch;
}
extern gt_time_t gt_time(gt_time_t *arg)
{
time_t time_now;
time(&time_now);
if (arg)
*arg = time_now;
return time_now;
}
extern struct tm *gt_gmtime(const gt_time_t *timeptr)
{
time_t time_now = time(NULL);
struct tm *tm = gmtime(&time_now);
gt_time_t days_since_epoch = *timeptr + gt_epoch_adjustment(NULL);
// Re-construct struct tm
tm->tm_sec = days_since_epoch % 60;
days_since_epoch /= 60;
tm->tm_min = days_since_epoch % 60;
days_since_epoch /= 60;
tm->tm_hour = days_since_epoch % 24;
days_since_epoch /= 24;
civil_from_epoch_days(days_since_epoch, tm);
tm->tm_wday = weekday_from_epoch_days(days_since_epoch);
bool is_leap_year;
if (tm->tm_year % 4 == 0)
if (tm->tm_year % 100 == 0)
if (tm->tm_year % 400 == 0)
is_leap_year = true;
else
is_leap_year = false;
else
is_leap_year = true;
else
is_leap_year = false;
static const unsigned year_days_before_month[12] = { 0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335 };
if (is_leap_year || tm->tm_mon <= 1)
tm->tm_yday = year_days_before_month[12] + tm->tm_mday;
else
tm->tm_yday = year_days_before_month[12] + tm->tm_mday - 1;
tm->tm_yday--; // struct tm counts day-of-year from 0
tm->tm_isdst = 0;
return tm;
}
extern struct tm *gt_localtime(gt_time_t *time);
extern gt_time_t gt_mktime(struct tm *tm);