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.^[1] 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.^[2][3] In 1989, the ANSI C standard is released including strftime and other date and time functions.^[4]

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[5]
	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
	struct timespec	time with seconds and nanoseconds

The timespec and related types were originally proposed by Markus Kuhn to provide a variety of time bases, but only TIME_UTC was accepted.^[6] 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.

#include <time.h>
#include <stdlib.h>
#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);
}

The output is:

Current time is Thu Sep 15 21:18:23 2016

See also

• Unix time
• Year 2038 problem

References

1. ISO/IEC 9899:1999 specification (PDF). p. 351, § 7.32.2.
2. "PWB1 date system command - man page". www.tuhs.org.
3. "date.c sourcecode of PWB1". www.tuhs.org.
4. "Rationale for American National Standard for Information Systems - Programming Language - C - Date and Time". www.lysator.liu.se.
5. open-std.org - Committee Draft -- May 6, 2005 page 355
6. Markus Kuhn. "Modernized API for ISO C". cl.cam.ac.uk.

External links