Header File  

time.h  

Category  

Time and Date Routines 

Prototype  

time_t mktime(struct tm *t);  

Description  

Converts time to calendar format. 

Converts the time in the structure pointed to by t into a calendar time with the same format used by the time function. The original values of the fields tm_sec, tm_min, tm_hour, tm_mday, and tm_mon are not restricted to the ranges described in the tm structure. If the fields are not in their proper ranges, they are adjusted. Values for fields tm_wday and tm_yday are computed after the other fields have been adjusted.  

The tm_isdst (Daylight Savings Time) field is adjusted with the correct value after calling the function mktime. Also, tm_isdst is used for adusting the value of tm_hour. For example, if the value of tm_isdst is 1, but the current date is not DST, mktime updates tm_hour (tm_hour = tm_hour - 1) and sets tm_isdst to 0.  

The allowable range of calendar times is Jan 1 1970 00:00:00 to Jan 19 2038 03:14:07. 

Return Value  

On success, mktime returns calendar time as described above. 

On error (if the calendar time cannot be represented), mktime returns -1. 

Example  

#include <stdio.h> #include <time.h>
        char *wday[] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday",
        "Saturday", "Unknown"}; int main(void) { struct tm time_check; int year, month, day; /*
        Input a year, month and day to find the weekday for */ printf("Year: "); scanf("%d",
        &year); printf("Month: "); scanf("%d", &month); printf("Day: "); scanf("%d",
        &day); /* load the time_check structure with the data */ time_check.tm_year = year -
        1900; time_check.tm_mon = month - 1; time_check.tm_mday = day; time_check.tm_hour = 0;
        time_check.tm_min = 0; time_check.tm_sec = 1; time_check.tm_isdst = -1; /* call mktime to
        fill in the weekday field of the structure */ if (mktime(&time_check) == -1)
        time_check.tm_wday = 7; /* print out the day of the week */ printf("That day is a %s\n",
        wday[time_check.tm_wday]); return 0; } 

Portability