defined in header file time.h

time_t mktime( struct tm * tm_ptr);

The mktime() function converts tm structure to time_t as local time.

The value in the tm_isdst field informs mktime() function to consider day light saving time (DST) for the time supplied in the tm_ptr.

The mktime() function modifies the supplied tm_ptr for fields tm_wday (day of the week) and tm_yday (day in the year) and sets the values according to the values in other fields.

/*tm structure */
struct tm 
	int tm_sec;         /* seconds */
	int tm_min;         /* minutes */
	int tm_hour;        /* hours */
	int tm_mday;        /* day of the month */
	int tm_mon;         /* month */
	int tm_year;        /* year */
	int tm_wday;        /* day of the week */
	int tm_yday;        /* day in the year */
	int tm_isdst;       /* daylight saving time */

tm_ptrpointer to tm instance with local calendar time to convert.

time_t value corresponding to the calendar time (struct tm) passed as argument. on error, it returns -1.

example program

#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h> /* memset */

//  gcc -o mktime_prg mktime_prg.c
// ./mktime_prg
int main () 
	printf("started program %s-%d\n", __FUNCTION__, __LINE__);
	struct tm tm_data;
	memset( &tm_data, 0, sizeof(struct tm));
	tm_data.tm_year = 2019 - 1900;
	tm_data.tm_mon = 3 - 1;
	tm_data.tm_mday = 19;
	tm_data.tm_hour = 11;
	tm_data.tm_min = 13;
	tm_data.tm_sec = 45;
	time_t time = mktime( &tm_data);
	printf(" time=%ld \n day-of-the-week=%d \n day-in-the-year=%d \n", time, tm_data.tm_wday, tm_data.tm_yday);

program output

started program main-10


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