Time representation

Names specified for `<time.h>`, “Time representation”
Name	Description	Notes				Source	Availability
`asctime()`	Convert time to string				(·)	`<time.h>`	C89	C90	C95	C99	C11
`asctime_s()`	Convert time to string				(·)	`<time.h>`					C11
`CLOCKS_PER_SEC`	Granularity of execution-time clock			M		`<time.h>`	C89	C90	C95	C99	C11
`clock()`	Get execution time				(·)	`<time.h>`	C89	C90	C95	C99	C11
`clock_t`	Execution-time type				T	`<time.h>`	C89	C90	C95	C99	C11
`ctime()`	Convert time to string				(·)	`<time.h>`	C89	C90	C95	C99	C11
`ctime_s()`	Convert time to string				(·)	`<time.h>`					C11
`difftime()`	Compare times				(·)	`<time.h>`	C89	C90	C95	C99	C11
`errno_t`	Error-number type		?		T	Headers					C11
`gmtime()`	Convert calendar time to UTC				(·)	`<time.h>`	C89	C90	C95	C99	C11
`gmtime_s()`	Convert calendar time to UTC				(·)	`<time.h>`					C11
`localtime()`	Convert calendar time to local time				(·)	`<time.h>`	C89	C90	C95	C99	C11
`localtime_s()`	Convert calendar time to local time				(·)	`<time.h>`					C11
`mktime()`	Create calendar time				(·)	`<time.h>`	C89	C90	C95	C99	C11
`NULL`	Null pointer constant	L		M		Headers	C89	C90	C95	C99	C11
`rsize_t`	Implementation-defined size type with implied run-time constraints	L	?		T	Headers					C11
`size_t`	Implementation-defined size type	H			T	Headers	C89	C90	C95	C99	C11
`strftime()`	Convert time to string				(·)	`<time.h>`	C89	C90	C95	C99	C11
`struct timespec`	Nanosecond time type				T	`<time.h>`					C11
`struct tm`	Time type				T	Headers	C89	C90	C95	C99	C11
`TIME_UTC`	Indicator for UTC time in `struct timespec`			M		`<time.h>`					C11
`time()`	Get calendar time				(·)	`<time.h>`	C89	C90	C95	C99	C11
`time_t`	Calendar-time type				T	`<time.h>`	C89	C90	C95	C99	C11
`timespec_get()`	Read nanosecond time				(·)	`<time.h>`					C11

This header is available in C89, C90, C95, C99 and C11.

C supports several concepts of time. Program duration is available, indicating how long the program has been running. Real time is available too, in both arithmetic (timezone-independent) and structural forms, with functions to convert them to strings. Nanosecond timing is available for threading operations.

Most of these facilities are available in <time.h>.