C in a Nutshell

Name

mktime

Synopsis

Determines the time represented by a struct tm value

#include <time.h>
time_tmktime( struct tm *timeptr );

The mktime() function calculates the local calendar time represented by the member values in the object referenced by the pointer argument.

The type struct tm is defined in time.h as follows:

struct tm {
  int tm_sec;         /* Seconds (0-60; 1 leap second) */
  int tm_min;         /* Minutes (0-59) */
  int tm_hour;        /* Hours   (0-23) */
  int tm_mday;        /* Day     (1-31) */
  int tm_mon;         /* Month   (0-11) */
  int tm_year;        /* Year    (difference from 1900) */
  int tm_wday;        /* Day of week (0-6)   */
  int tm_yday;        /* Day of year (0-365) */
  int tm_isdst;       /* Daylight saving time (-1, 0, 1) */
};

The member tm_isdst is equal to 0 if daylight saving time is not in effect, or 1 if it is. A negative value indicates that the information is not available, in which case mktime() attempts to calculate whether daylight saving time is applicable at the time represented by the other members.

The mktime() function ignores the tm_wday and tm_yday members in determining the time, but does use tm_isdst. The other members may contain values outside their normal ranges. Once it has calculated the time represented, mktime() adjusts the struct tm members so that each one is within its normal range, and also sets tm_wday and tm_yday accordingly. The return value is the number of seconds from the epoch (usually midnight on January 1, 1970, UTC) to the time represented in the structure, or -1 to indicate an error.

Example

 time_t ...