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C date and time functions

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C standard library (libc)
General topics
Miscellaneous headers

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 timespec and related types were originally proposed by Markus Kuhn to provide a variety of time bases, but only TIME_UTC 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. 

#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

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.

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