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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 (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:

The process is not superuser.