I have a string containing a local date/time and I need to convert it to a time_t value (in UTC) - I've been trying this:
char* date = "2009/09/01/00";
struct tm cal = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, NULL};
strptime(date, "%Y/%m/%d/%H", &cal);
time_t t = mktime(&cal);
but the time_t value I get back is the value that I would expect if the string was being parsed as UTC not local time. Maybe I have misunderstood what strptime is supposed to do, but in my timezone (UK) on the 1st September we are using BST (ie UTC + 1 hour) so I would expect the value I end up with to be 1 hour ahead of UTC.
Is there a way to interpret the string as localtime, automatically taking into account the UTC offset that would have been in effect on that date? Note that I need the time_t value not a struct tm, in the example above I want the time_t value to correspond to 2009-09-01 01:00:00 GMT
You can use mktime
to interpret a struct tm in the local timezone. When you do so, be careful to set the tm_isdst
flag. It's 0 for summertime, 1 for wintertime, and to -1 to have mktime
figure it out. Here's some example code:
void main()
{
char* date = "2009/09/01/00";
struct tm cal = {};
// Read string into struct tm
strptime(date, "%Y/%m/%d/%H", &cal);
// Tell mktime to figure out the daylight saving time
cal.tm_isdst = -1;
printf("%20s: %s", "Before mktime", asctime(&cal));
// Convert struct tm to time_t
time_t t = mktime(&cal);
// Convert time_t to localtime
struct tm localcal = *localtime(&t);
printf("%20s: %s", "Local time", asctime(&localcal));
printf("%20s: %i\n", "Local DST", localcal.tm_isdst);
// Convert time_t to GMT
struct tm gmcal = *gmtime(&t);
printf("%20s: %s", "GM time", asctime(&gmcal));
printf("%20s: %i\n", "GM DST", gmcal.tm_isdst);
}
This prints (I live in GMT+1, and it's wintertime now):
Before mktime: Tue Sep 1 00:00:00 2009
Local time: Tue Sep 1 00:00:00 2009
Local DST: 1
GM time: Mon Aug 31 22:00:00 2009
GM DST: 0
It looks like mktime
converts a date in September based on the current daylight savings time. It's November now, so it's actually one hour off. I haven't found a way to correct that.
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