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