tm

Structure holding a calendar date and time broken down into its components.

int tm_sec	seconds after the minute [0, 61](until C99) / [0, 60] (since C99)^{[note 1]}
int tm_min	minutes after the hour [0, 59]
int tm_hour	hours 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	Daylight Saving Time flag. The value is positive if DST is in effect, zero if not and negative if no information is available

Range allows for a positive leap second. Two leap seconds in the same minute are not allowed (the C90 range 0..61 was a defect)

Show the start of calendar time.

Run this code

#include <stdio.h>
#include <time.h>

int main(void)
{
    struct tm start = {.tm_mday=1};
    mktime(&start);
    printf("%s\n", asctime(&start));
}

Possible output:

Mon Jan  1 00:00:00 1900

localtimelocaltime_s (C11)	converts time since epoch to calendar time expressed as local time (function)
gmtimegmtime_s (C11)	converts time since epoch to calendar time expressed as Coordinated Universal Time (UTC) (function)