mktime(3C)

mktime -- converts a tm structure to a calendar time

Synopsis

   #include <time.h>
   
   time_t mktime (struct tm timeptr);

Description

mktime converts the time represented by the tm structure pointed to by timeptr into a calendar time (the number of seconds since 00:00:00 UTC, January 1, 1970).

The tm structure has the following format.

   struct	tm {	
   	int	tm_sec;	/ seconds after the minute [0, 61]  /
   	int	tm_min;	/ minutes after the hour [0, 59] /
   	int	tm_hour;	/ hour since midnight [0, 23] /
   	int	tm_mday;	/ day of the month [1, 31] /
   	int	tm_mon;	/ months since January [0, 11] /
   	int	tm_year;	/ years since 1900 /
   	int	tm_wday;	/ days since Sunday [0, 6] /
   	int	tm_yday;	/ days since January 1 [0, 365] /
   	int	tm_isdst;	/ flag for daylight savings time /
   };

In addition to computing the calendar time, mktime normalizes the supplied tm structure. The original values of the tm_wday and tm_yday components of the structure are ignored, and the original values of the other components are not restricted to the ranges indicated in the definition of the structure. On successful completion, the values of the tm_wday and tm_yday components are set appropriately, and the other components are set to represent the specified calendar time, but with their values forced to be within the appropriate ranges. The final value of tm_mday is not set until tm_mon and tm_year are determined.

The original values of the components may be either greater than or less than the specified range. For example, a tm_hour of -1 means 1 hour before midnight, tm_mday of 0 means the day preceding the current month, and tm_mon of -2 means 2 months before January of tm_year.

If tm_isdst is positive, the original values are assumed to be in the alternate timezone. If it turns out that the alternate timezone is not valid for the computed calendar time, then the components are adjusted to the main timezone. Likewise, if tm_isdst is zero, the original values are assumed to be in the main timezone and are converted to the alternate timezone if the main timezone is not valid. If tm_isdst is negative, the correct timezone is determined and the components are not adjusted.

Local timezone information is used as if mktime had called tzset.

mktime returns the specified calendar time. If the calendar time cannot be represented, the function returns the value (time_t)-1.

Usage

What day of the week is July 4, 2001?

   	#include <stdio.h>
   	#include <time.h>
   
   	static char const wday[] = {
   		"Sunday", "Monday", "Tuesday", "Wednesday",
   		"Thursday", "Friday", "Saturday", "-unknown-"
   	};
   	struct tm time_str;
   	/.../
   	time_str.tm_year	= 2001 - 1900;
   	time_str.tm_mon	= 7 - 1;
   	time_str.tm_mday	= 4;
   	time_str.tm_hour	= 0;
   	time_str.tm_min	= 0;
   	time_str.tm_sec		= 1;
   	time_str.tm_isdst	= -1;
   	if (mktime(&time_str)== -1)
   	    time_str.tm_wday=7;
   	printf("%s\n", wday[time_str.tm_wday]);

References

ctime(3C), getenv(3C), timezone(4)

Notices

tm_year of the tm structure must be for year 1970 or later. Calendar times before 00:00:00 UTC, January 1, 1970 or after 03:14:07 UTC, January 19, 2038 cannot be represented.