time.h — Time and date

The time.h header file declares the time and date functions:

asctime()

clock()

ctime()

difftime()

gmtime()

localtime()

mktime()

strftime()

strptime()

time()

tzset()1

Note:

This function is supported only in a POSIX program.

The time.h header file also provides:

A structure timespec that contains the following members:

time_t     tv_sec;            seconds
long       tv_nsec;           nanoseconds

A structure timespec64 that contains the following members:
```
time64_t     tv_sec;          seconds
long         tv_nsec;         nanoseconds
```
Note: In addition to the requirements for accessing structure timespec, feature test macro _LARGE_TIME_API is needed to expose timespec64.
A structure tm that contains the components of a calendar time. See Table 1 for a list of the members of the tm structure. This structure is used by the functions asctime(), gmtime(), localtime(), mktime(), strftime(), and strptime().
A macro CLOCKS_PER_SEC equal to the number per second of the value returned by the clock() function.
Types clock_t, time_t, and size_t.
The NULL pointer constant. For more information on NULL and the type size_t, see stddef.h — typedef statements.
The macro CLK_TCK, which is the number of clock ticks per second, is kept for historical reasons. It was used in connection with the return value of the clock() function. CLK_TCK has been withdrawn and is not supported as part of Single UNIX Specification, Version 3. New applications should use sysconf(_SC_CLK_TCK) instead of the CLK_TCK macro.

However, if it is necessary to continue using this symbol in an application written for Single UNIX Specification, Version 3, define the feature test macro _UNIX03_WITHDRAWN before including any standard system headers. The macro exposes all interfaces and symbols removed in Single UNIX Specification, Version 3.

Table 1. Fields of tm Structure
Field	Description
tm_sec	Seconds (0-60)
tm_min	Minutes (0-59)
tm_hour	Hours (0-23)
tm_mday	Day of month (1-31)
tm_mon	Month (0-11; January = 0)
tm_year	Year (current year minus 1900)
tm_wday	Day of week (0-6; Sunday = 0)
tm_yday	Day of year (0-365; January 1 = 0)
tm_isdst	Zero if Daylight Saving Time is not in effect; positive if Daylight Saving Time is in effect; negative if the information is not available.

The time functions are affected by the current locale selected. The LC_CTYPE category affects the behavior of the strftime(), strptime(), and wcsftime() functions. The LC_TOD category affects the behavior of the gmtime(), mktime(), and localtime() functions.