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]} (public member object)
int tm_min	minutes after the hour – [0, 59] (public member object)
int tm_hour	hours since midnight – [0, 23] (public member object)
int tm_mday	day of the month – [1, 31] (public member object)
int tm_mon	months since January – [0, 11] (public member object)
int tm_year	years since 1900 (public member object)
int tm_wday	days since Sunday – [0, 6] (public member object)
int tm_yday	days since January 1 – [0, 365] (public member object)
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 (public member object)

↑ 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)
{
    printf("%s\n", asctime(&(struct tm){.tm_mday=1}));
}

Possible output:

Sun Jan  1 00:00:00 1900

localtime	converts time since epoch to calendar time expressed as local time (function)
gmtime	converts time since epoch to calendar time expressed as Coordinated Universal Time (UTC) (function)
C++ documentation for tm

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