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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.

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]

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

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++ with the release of C++20 in std::chrono.

The following C source code prints the current time to the standard output stream.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(void) {
    // Obtain current time.
    time_t current_time = time(NULL);

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

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

    if (!time_string) {
        fprintf(stderr, "Failure to convert the current time.\n");
        return EXIT_FAILURE;
    }

    // Print to stdout. ctime() has already added a terminating newline character.
    printf("Current time is %s", time_string);
    return EXIT_SUCCESS;
}

The output is:

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

• Unix time
• Year 2038 problem