Weird mktime logic with negative seconds

Question

I've been using mktime/localtime for time management, including some heavy arithmetic on dates/times.

I noticed something very weird when providing to mktime a struct tm that contains negative values.

Take the code below. There was a DST change in LA on Nov 3rd, 2013. If I specify time in tm as 2013-11-04 midnight and subtract 24 hours, I get the same value as 2013-11-03 midnight. It is 25 hours difference UTC-wise, which is fine, as with isdst=-1 one could say we're looking at 'wallclock-time'. Same if I subtract 1440 minutes (24*60). But, if I subtract 86400 (24*60*60) seconds, I get 2013-11-03 1am. That is 24 hours difference UTC-wise. Here's the output from the code below:

2013-11-03 00:00:00 (gmtoff=0, isdst=-1) -> 2013-11-03 00:00:00 (gmtoff=-25200, isdst=1) -> 1383462000
2013-12--27 00:00:00 (gmtoff=0, isdst=-1) -> 2013-11-03 00:00:00 (gmtoff=-25200, isdst=1) -> 1383462000
2013-11-04 -24:00:00 (gmtoff=0, isdst=-1) -> 2013-11-03 00:00:00 (gmtoff=-25200, isdst=1) -> 1383462000
2013-11-04 00:-1440:00 (gmtoff=0, isdst=-1) -> 2013-11-03 00:00:00 (gmtoff=-25200, isdst=1) -> 1383462000
2013-11-04 00:00:-86400 (gmtoff=0, isdst=-1) -> 2013-11-03 01:00:00 (gmtoff=-25200, isdst=1) -> 1383465600

For me it doesn't make sense - why are seconds treated differently than minutes, hours and days? I looked at man and the C standard but couldn't find anything.

This behavior breaks some of my assumptions and complicates things. Does someone know a good alternative to mktime/localtime (I tested boost, ICU and tzcode, all too slow for what I need).

Thanks in advance for any thoughts :)

#include <time.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
char* printtm(struct tm tm)
{
  static char buf[100];
  sprintf(buf, "%04d-%02d-%02d %02d:%02d:%02d (gmtoff=%ld, isdst=%d)",
    tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday,
    tm.tm_hour, tm.tm_min, tm.tm_sec,
    tm.tm_gmtoff, tm.tm_isdst);
  return buf;
}

void test(int y, int m, int d, int hh, int mm, int ss, int isdst)
{
  struct tm tm;
  memset(&tm, 0, sizeof(tm));
  tm.tm_year = y - 1900;
  tm.tm_mon = m - 1;
  tm.tm_mday = d;
  tm.tm_hour = hh;
  tm.tm_min = mm;
  tm.tm_sec = ss;
  tm.tm_isdst = isdst;
  printf("%s -> ", printtm(tm));
  time_t t = mktime(&tm);
  printf("%s -> %ld\n", printtm(tm), t);
}


int main()
{
  setenv("TZ", ":America/Los_Angeles", 1);
  tzset();

  test(2013,11,03, 0,0,0, -1);
  test(2013,12,-27, 0,0,0, -1);
  test(2013,11,04, -24,0,0, -1);
  test(2013,11,04, 0,-1440,0, -1);
  test(2013,11,04, 0,0,-86400, -1);

  return 0;
}

Answer 1

Using mktime with out-of-range values in the struct tm and tm_isdst==-1 is problematic and under-specified. Personally, I think the way your system is behaving here is wrong, but the standard isn't clear on how it's supposed to behave, and thus any such usage is non-portable at best. To do arithmetic on struct tm, you should make sure tm_isdst is set to either 0 or 1 beforehand, so that the result of unambiguous.

Note that one easy way to do this is to simply call mktime on the original struct tm (with tm_isdst==-1) before applying the arithmetic to determine whether daylight time is in effect (i.e. to fill in a definitive value for tm_isdst) then call it again after making your arithmetic adjustments.