C date and time functions

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.

History

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.

Overview of functions

The C date and time operations are defined in the time.h header file (ctime header in C++).

	Identifier	Description
Time manipulation	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
Format conversions	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 <syntaxhighlight lang="text" class="" style="" inline="1">timespec</syntaxhighlight> and related types were originally proposed by Markus Kuhn to provide a variety of time bases, but only <syntaxhighlight lang="text" class="" style="" inline="1">TIME_UTC</syntaxhighlight> was accepted. The functionalities were, however, added to C++ in 2020 in std::chrono.

Example

The following C source code prints the current time to the standard output stream. <syntaxhighlight lang="c">

1. include <time.h>
2. include <stdlib.h>
3. include <stdio.h>

int main(void) {

   time_t current_time;
   char* c_time_string;

   /* Obtain current time. */
   current_time = time(NULL);

   if (current_time == ((time_t)-1))
   {
       (void) fprintf(stderr, "Failure to obtain the current time.\n");
       exit(EXIT_FAILURE);
   }

   /* Convert to local time format. */
   c_time_string = ctime(&current_time);

   if (c_time_string == NULL)
   {
       (void) fprintf(stderr, "Failure to convert the current time.\n");
       exit(EXIT_FAILURE);
   }

   /* Print to stdout. ctime() has already added a terminating newline character. */
   (void) printf("Current time is %s", c_time_string);
   exit(EXIT_SUCCESS);

}

</syntaxhighlight>

The output is: <syntaxhighlight lang="output"> Current time is Thu Sep 15 21:18:23 2016 </syntaxhighlight>

See also

• Unix time
• Year 2038 problem