C
#include <time.h>
struct tm *localtime(const time_t *timep);
time_t mktime(struct tm *tm);

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 */
};

The mktime() function modifies the fields of the tm structure as fol‐
lows: tm_wday and tm_yday are set to values determined from the con‐
tents of the other fields; if structure members are outside their valid
interval, they will be normalized (so that, for example, 40 October is
changed into 9 November); tm_isdst is set (regardless of its initial
value) to a positive value or to 0, respectively, to indicate whether
DST is or is not in effect at the specified time. Calling mktime()
also sets the external variable tzname with information about the cur‐
rent timezone.