Converting string containing localtime into UTC in C - c

Question

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

Answer 1

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.

Answer 2

Here's my version, using tm_gmtoff. Hopefully, the library takes care of daylight savings time ...
#define _BSD_SOURCE
#define _XOPEN_SOURCE
#include <stdio.h>
#include <time.h>
int gmtoffset(void) {
struct tm *dummy;
time_t t = 0;
dummy = localtime(&t);
return dummy->tm_gmtoff; /* _BSD_SOURCE */
}
int main(void) {
int off;
const char *date = "2009/09/01/00";
struct tm cal = {0};
time_t t;
off = gmtoffset();
strptime(date, "%Y/%m/%d/%H", &cal); /* _XOPEN_SOURCE */
t = mktime(&cal);
printf("t --> %s", ctime(&t)); /* ctime includes a final '\n' */
t -= off;
printf("t-off --> %s", ctime(&t));
return 0;
}
$ /usr/bin/gcc ptime.c
$ ./a.out
t --> Tue Sep 1 01:00:00 2009
t-off --> Tue Sep 1 00:00:00 2009

Answer 3

I think I've cracked it now, thanks to Andomar - this code does what I need and appears to work regardless of the current DST status (I changed the clock on my PC to check this):
#include <time.h>
#include <assert.h>
time_t parseLocalDate(char* date){
struct tm cal = {0, 0, 0, 0, 0, 0, 0, 0, -1, 0, NULL};
strptime(date, "%Y/%m/%d/%H", &cal);
return mktime(&cal);
}
int main(int argc, char *argv[]){
// DST is effect, Local Time = GMT+1
assert(1251759600 == parseLocalDate("2009/09/01/00")); // Mon, 31 Aug 2009 23:00:00 GMT
assert(1254351600 == parseLocalDate("2009/10/01/00")); // Wed, 30 Sep 2009 23:00:00 GMT
// DST not in effect, Local Time = GMT
assert(1257033600 == parseLocalDate("2009/11/01/00")); // Sun, 01 Nov 2009 00:00:00 GMT
}