Converting Between Local Times and GMT/UTC in C/C++

Question

What's the best way to convert datetimes between local time and UTC in C/C++?

By "datetime", I mean some time representation that contains date and time-of-day. I'll be happy with time_t, struct tm, or any other representation that makes it possible.

My platform is Linux.

Here's the specific problem I'm trying to solve: I get a pair of values containing a julian date and a number of seconds into the day. Those values are in GMT. I need to convert that to a local-timezone "YYYYMMDDHHMMSS" value. I know how to convert the julian date to Y-M-D, and obviously it is easy to convert seconds into HHMMSS. However, the tricky part is the timezone conversion. I'm sure I can figure out a solution, but I'd prefer to find a "standard" or "well-known" way rather than stumbling around.

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A possibly related question is Get Daylight Saving Transition Dates For Time Zones in C

Answer 1

How do you know which timezone the data was in at the time it was recorded? Does it indicate EDT vs EST (etc.)?

Answer 2

You're supposed to use combinations of gmtime/localtime and timegm/mktime. That should give you the orthogonal tools to do conversions between struct tm and time_t.

For UTC/GMT:

time_t t;
struct tm tm;
struct tm * tmp;
...
t = timegm(&tm);
...
tmp = gmtime(t);

For localtime:

t = mktime(&tm);
...
tmp = localtime(t);

All tzset() does is set the internal timezone variable from the TZ environment variable. I don't think this is supposed to be called more than once.

If you're trying to convert between timezones, you should modify the struct tm's tm_gmtoff.

Answer 3

If you need to worry about converting date/time with timezone rules, you might want to look into ICU.

Answer 4

If on Windows, you don't have timegm() available to you:

struct tm *tptr;
time_t secs, local_secs, gmt_secs;
time( &secs );  // Current time in GMT
// Remember that localtime/gmtime overwrite same location
tptr = localtime( &secs );
local_secs = mktime( tptr );
tptr = gmtime( &secs );
gmt_secs = mktime( tptr );
long diff_secs = long(local_secs - gmt_secs);

or something similar...