Odd behavior of mktime()

Question

Continuing on my attempt to create a DateTime class , I am trying to store the "epoch" time in my function:

void DateTime::processComponents(int month, int day, int year, 
                                 int hour, int minute, int second) {
    struct tm time;
    time.tm_hour = hour;
    time.tm_min = minute;
    time.tm_sec = second;
    time.tm_mday = day;
    time.tm_mon = month;
    time.tm_year = year - 1900;
    ticks_ = mktime(&time);

    processTm(time);
}

void DateTime::processTm(struct tm time) {
    second_ = time.tm_sec;
    minute_ = time.tm_min;
    hour_ = time.tm_hour;
    weekday_ = time.tm_wday;
    monthday_ = time.tm_mday;
    yearday_ = time.tm_yday;
    month_ = time.tm_mon;
    year_ = time.tm_year + 1900;
}

For an arbitrary date, processComponents(5,5,1990,1,23,45) (June 6, 1990 1:23:45 am), it sets all values correctly and as expected.

However, upon further testing, I find that for processComponents(0,0,1970,0,0,0) (January 1, 1970, 12:00:00 am), mktime(&time) causes time to be screwed up:

time.tm_mon  = 11;
time.tm_mday = 30;
time.tm_year = 69;
time.tm_hour = 23;
time.tm_min  = 0;
time.tm_sec  = 0;

time.tm_isdst  = 0;
time.tm_gmtoff = -18000;
time.tm_zone   = "EST";
time.tm_wday   = 2;
time.tm_yday   = 363;

Translating to a date of December 31, 1969 11:00:00 pm.

I can verify that mktime() is responsible, because by commenting out that line, it reports the date and time correctly as January 1, 1970 12:00:00 am.

Why is mktime() only messing up the epoch? And how should I fix / workaround this?

Thanks!

score 5 · Accepted Answer · edited May 23 '17 at 12:29

You're passing 0 as the day parameter and putting that into time.tm_mday. That component (and only that component) of struct tm is 1-based, not 0-based.

Don't ask me why.

To specify 01 Jan 1970, 12:00:00am you'd want to call it like so:

processComponents(0,1,1970,0,0,0);

And as sdtom mentioned, you'll want to make sure that tm_isdst is set appropriately - 0 for not in effect, positive for in effect, and negative for you don't know (in which case mktime() should try to guess).

Just to let you know, when I pass the date you have (0 Jan 1970, 00:00:00) to mktime() in MSVC 9 it returns an error (the passed in struct tm is untouched and the returned time_t value is -1).

Thanks! When I explicitly set `time.tm_isdst = -1` everything works. — Austin Hyde, Nov 08 '09 at 07:29

score 3 · Answer 2 · answered Nov 08 '09 at 04:52

3

Since it is off by one hour I would expect daylight savings time. Is the value of time.tm_isdst getting set somewhere? If you aren't setting it, it could be randomly getting set to 1 or 0 which would affect your results.

answered Nov 08 '09 at 04:52

sdtom

890
4
8

+1. I was thinking along the same lines. According to Python (on Mac OS X), `gmtime(0)` returns: `time.struct_time(tm_year=1970, tm_mon=1, tm_mday=1, tm_hour=0, tm_min=0, tm_sec=0, tm_wday=3, tm_yday=1, tm_isdst=0)`. I'm not sure what that proves, though, because Python does some translation of `struct tm` when it builds a `time.struct_time` object. – Daniel Pryden Nov 08 '09 at 04:57
`tm_isdst` is reporting 0. I have updated the post to reflect the rest of the `time` structure, according to my debugger. – Austin Hyde Nov 08 '09 at 05:24
specify -1 for it as you are not setting it anywhere and don't know it :) – rama-jka toti Nov 08 '09 at 10:41

score 0 · Answer 3 · answered Dec 21 '12 at 02:27

Passing all zeros to mktime() is interpreted as "Sun Jan 0 00:00:00 1900". Based on this, there needs to be some adjustments...

// the input is local time
// the output is seconds since the epoch
// The epoch is Jan 1, 1970 @ 0:00 GMT
time_t mktime_wrapper( int month, int day, int year,
                       int hour=0, int min=0, int sec=0, bool isDST=-1
                     )
   {
   tm t;
   t.tm_sec=sec, t.tm_min=min, t.tm_hour=hour, t.tm_isdst=isDST;
   t.tm_mday=day, t.tm_mon=month-1, t.tm_year=year-1900;
   return mktime( &t );
   }

Odd behavior of mktime()

3 Answers3