#### 21.4.1 Simple Calendar Time

This section describes the `time_t` data type for representing calendar time as simple time, and the functions which operate on simple time objects. These facilities are declared in the header file `time.h`.

— Data Type: time_t

> This is the data type used to represent simple time. Sometimes, it also represents an elapsed time. When interpreted as a calendar time value, it represents the number of seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal Time. (This calendar time is sometimes referred to as the epoch.) POSIX requires that this count not include leap seconds, but on some systems this count includes leap seconds if you set `TZ` to certain values (see TZ Variable).

> Note that a simple time has no concept of local time zone. Calendar Time T is the same instant in time regardless of where on the globe the computer is.

> In the GNU C Library, `time_t` is equivalent to `long int`. In other systems, `time_t` might be either an integer or floating-point type.

The function `difftime` tells you the elapsed time between two simple calendar times, which is not always as easy to compute as just subtracting. See Elapsed Time.

— Function: time_t time (time_t *result)

> The `time` function returns the current calendar time as a value of type `time_t`. If the argument result is not a null pointer, the calendar time value is also stored in `*`result. If the current calendar time is not available, the value `(time_t)(-1)` is returned.

— Function: int stime (const time_t *newtime)

> `stime` sets the system clock, i.e., it tells the system that the current calendar time is newtime, where `newtime` is interpreted as described in the above definition of `time_t`.

> `settimeofday` is a newer function which sets the system clock to better than one second precision. `settimeofday` is generally a better choice than `stime`. See High-Resolution Calendar.

> Only the superuser can set the system clock.

> If the function succeeds, the return value is zero. Otherwise, it is `-1` and `errno` is set accordingly:

> `EPERM`
>   The process is not superuser.