This is time/calendar_time.c (Listing 10-1, page 191), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* calendar_time.c

   Demonstrate the use of functions for working with calendar time.

   This program retrieves the current time and displays it in various forms.
#include <locale.h>
#include <time.h>
#include <sys/time.h>
#include "tlpi_hdr.h"

#define SECONDS_IN_TROPICAL_YEAR (365.24219 * 24 * 60 * 60)
main(int argc, char *argv[])
    time_t t;
    struct tm *gmp, *locp;
    struct tm gm, loc;
    struct timeval tv;

    /* Retrieve time, convert and display it in various forms */

    t = time(NULL);
    printf("Seconds since the Epoch (1 Jan 1970): %ld", (long) t);
    printf(" (about %6.3f years)\n", t / SECONDS_IN_TROPICAL_YEAR);

    if (gettimeofday(&tv, NULL) == -1)
    printf("  gettimeofday() returned %ld secs, %ld microsecs\n",
            (long) tv.tv_sec, (long) tv.tv_usec);

    gmp = gmtime(&t);
    if (gmp == NULL)

    gm = *gmp;          /* Save local copy, since *gmp may be modified
                           by asctime() or gmtime() */
    printf("Broken down by gmtime():\n");
    printf("  year=%d mon=%d mday=%d hour=%d min=%d sec=%d ", gm.tm_year,
            gm.tm_mon, gm.tm_mday, gm.tm_hour, gm.tm_min, gm.tm_sec);
    printf("wday=%d yday=%d isdst=%d\n", gm.tm_wday, gm.tm_yday, gm.tm_isdst);

    /* The TZ environment variable will affect localtime().
       Try, for example:

                TZ=Pacific/Auckland calendar_time

    locp = localtime(&t);
    if (locp == NULL)

    loc = *locp;        /* Save local copy */

    printf("Broken down by localtime():\n");
    printf("  year=%d mon=%d mday=%d hour=%d min=%d sec=%d ",
            loc.tm_year, loc.tm_mon, loc.tm_mday,
            loc.tm_hour, loc.tm_min, loc.tm_sec);
    printf("wday=%d yday=%d isdst=%d\n\n",
            loc.tm_wday, loc.tm_yday, loc.tm_isdst);

    printf("asctime() formats the gmtime() value as: %s", asctime(&gm));
    printf("ctime() formats the time() value as:     %s", ctime(&t));

    printf("mktime() of gmtime() value:    %ld secs\n", (long) mktime(&gm));
    printf("mktime() of localtime() value: %ld secs\n", (long) mktime(&loc));



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