Header File 

time.h  

Category 

Time and Date Routines 

Prototype 

struct tm *gmtime(const time_t *timer); 

Description 

Converts date and time to Greenwich mean time (GMT). 

gmtime accepts the address of a value returned by time and returns a pointer to the structure of type tm containing the time elements. gmtime converts directly to GMT. 

The global long variable _timezone should be set to the difference in seconds between GMT and local standard time (in PST _timezone is 8 x 60 x 60). The global variable _daylight should be set to nonzero only if the standard U.S. daylight saving time conversion should be applied. 

This is the tm structure declaration from the time.h header file: 

struct tm { 

int tm_sec; /* Seconds */ 

int tm_min; /* Minutes */ 

int tm_hour; /* Hour (0 - 23) */ 

int tm_mday; /* Day of month (1 - 31) */ 

int tm_mon; /* Month (0 - 11) */ 

int tm_year; /* Year (calendar year minus 1900) */ 

int tm_wday; /* Weekday (0 - 6; Sunday is 0) */ 

int tm_yday; /* Day of year (0 -365) */ 

int tm_isdst; /* Nonzero if daylight saving time is in effect. */ 

}; 

These quantities give the time on a 24-hour clock, day of month (1 to 31), month (0 to 11), weekday (Sunday equals 0), year - 1900, day of year (0 to 365), and a flag that is nonzero if daylight saving time is in effect. 

Return Value 

gmtime returns a pointer to the structure containing the time elements. This structure is a static that is overwritten with each call. 

Example  

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
/* Pacific Standard Time & Daylight Savings */
char *tzstr = "TZ=PST8PDT"; 
int main(void)
{
   time_t t;
   struct tm *gmt, *area;
   putenv(tzstr);
   tzset();
   t = time(NULL);
   area = localtime(&t);
   printf("Local time is: %s", asctime(area));
   gmt = gmtime(&t);
   printf("GMT is:        %s", asctime(gmt));
   return 0;
}

Portability