C date and time functions

Library of C programs

The C date and time functions are a group of functions in the standard library of the C programming language implementing date and time manipulation operations. They provide support for time acquisition, conversion between date formats, and formatted output to strings.

The format string used in strftime traces back to at least PWB/UNIX 1.0, released in 1977. Its date system command includes various formatting options. In 1989, the ANSI C standard is released including strftime and other date and time functions.

The C date and time operations are defined in the time. h header file (ctime header in C++). Identifier Description Timemanipulation difftime computes the difference in seconds between two time t values time returns the current time of the system as a time t value, number of seconds, (which is usually time since an epoch, typically the Unix epoch). The value of the epoch is operating system dependent; 1900 and 1970 are often used. See RFC 868. clock returns a processor tick count associated with the process timespec get (C11) returns a calendar time based on a time base Formatconversions asctime converts a struct tm object to a textual representation (deprecated) ctime converts a time t value to a textual representation strftime converts a struct tm object to custom textual representation strptime converts a string with time information to a struct tm wcsftime converts a struct tm object to custom wide string textual representation gmtime converts a time t value to calendar time expressed as Coordinated Universal Time localtime converts a time t value to calendar time expressed as local time mktime converts calendar time to a time t value. Constants CLOCKS PER SEC number of processor clock ticks per second TIME UTC time base for UTC Types struct tm broken down calendar time type: year, month, day, hour, minute, second time t arithmetic time type (typically time since the Unix epoch) clock t process running time type timespec time with seconds and nanoseconds

The and related types were originally proposed by Markus Kuhn to provide a variety of time bases, but only was accepted. The functionalities were, however, added to C++ in 2020 in std::chrono.