21.4 Calendar Time

This section describes facilities for keeping track of calendar time. See Time Basics.

The GNU C library represents calendar time three ways:

• Simple time (the time_t data type) is a compact representation, typically giving the number of seconds of elapsed time since some implementation-specific base time.
• There is also a "high-resolution time" representation. Like simple time, this represents a calendar time as an elapsed time since a base time, but instead of measuring in whole seconds, it uses a struct timeval data type, which includes fractions of a second. Use this time representation instead of simple time when you need greater precision.
• Local time or broken-down time (the struct tm data type) represents a calendar time as a set of components specifying the year, month, and so on in the Gregorian calendar, for a specific time zone. This calendar time representation is usually used only to communicate with people.

• Simple Calendar Time: Facilities for manipulating calendar time.
• High-Resolution Calendar: A time representation with greater precision.
• Broken-down Time: Facilities for manipulating local time.
• High Accuracy Clock: Maintaining a high accuracy system clock.
• Formatting Calendar Time: Converting times to strings.
• Parsing Date and Time: Convert textual time and date information back into broken-down time values.
• TZ Variable: How users specify the time zone.
• Time Zone Functions: Functions to examine or specify the time zone.
• Time Functions Example: An example program showing use of some of the time functions.