#define _GNU_SOURCE
#include <time.h>

/* this is cut and paste from mktime. */

extern const short  __spm [];

time_t timegm(struct tm *const t) {
  register time_t  day;
  register time_t  i;
  register time_t years = t->tm_year - 70;

  if (t->tm_sec>60) { t->tm_min += t->tm_sec/60; t->tm_sec%=60; }
  if (t->tm_min>60) { t->tm_hour += t->tm_min/60; t->tm_min%=60; }
  if (t->tm_hour>60) { t->tm_mday += t->tm_hour/60; t->tm_hour%=60; }
  if (t->tm_mon>12) { t->tm_year += t->tm_mon/12; t->tm_mon%=12; }
  while (t->tm_mday>__spm[1+t->tm_mon]) {
    if (t->tm_mon==1 && __isleap(t->tm_year+1900)) {
      if (t->tm_mon==31+29) break;
      --t->tm_mday;
    }
    t->tm_mday-=__spm[t->tm_mon];
    ++t->tm_mon;
    if (t->tm_mon>11) { t->tm_mon=0; ++t->tm_year; }
  }

  if (t->tm_year < 70)
    return (time_t) -1;

  /* Days since 1970 is 365 * number of years + number of leap years since 1970 */
  day  = years * 365 + (years + 1) / 4;

  /* After 2100 we have to substract 3 leap years for every 400 years
     This is not intuitive. Most mktime implementations do not support
     dates after 2059, anyway, so we might leave this out for it's
     bloat. */
  if ((years -= 131) >= 0) {
    years /= 100;
    day -= (years >> 2) * 3 + 1;
    if ((years &= 3) == 3) years--;
    day -= years;
  }

  day += t->tm_yday = __spm [t->tm_mon] + t->tm_mday-1 + ( __isleap (t->tm_year+1900)  &  (t->tm_mon > 1) );

  /* day is now the number of days since 'Jan 1 1970' */
  i = 7;
  t->tm_wday = (day + 4) % i;                        /* Sunday=0, Monday=1, ..., Saturday=6 */

  i = 24;
  day *= i;
  i = 60;
  return ((day + t->tm_hour) * i + t->tm_min) * i + t->tm_sec;
}