<time.h>

The <time.h> include file declares the time and date functions:

Note: ¹ These functions are not available when LOCALETYPE(*CLD) is specified on the compilation command.

The <time.h> include file also provides:

A structure tm that contains the components of a calendar time. See gmtime() — Convert Time for a list of the tm structure members.
A macro CLOCKS_PER_SEC equal to the number per second of the value that is returned by the clock function.
Types clock_t, time_t, time64_t, and size_t.
The NULL pointer value.

For more information about NULL and the type size_t, see <stddef.h>.