time_and_date_in_c_v1.c

/*

    time_and_date_in_c_v1.c

C date and time:
https://en.wikipedia.org/wiki/C_date_and_time_functions
https://en.wikibooks.org/wiki/C_Programming/time.h/time_t
https://www.thegeekdiary.com/c-library/
http://www.qnx.com/developers/docs/6.5.0/index.jsp?topic=%2Fcom.qnx.doc.dinkum_en_c99%2Ftime.html
https://en.cppreference.com/w/c/chrono/time_t
http://zetcode.com/articles/cdatetime/
http://zetcode.com/gui/winapi/datetime/
https://c-for-dummies.com/blog/?p=3120

C function difftime()
https://www.tutorialspoint.com/c_standard_library/c_function_difftime.htm

Description
The C library function double difftime(time_t time1, time_t time2)
returns the difference of seconds between time1 and time2 i.e. (time1 - time2).
The two times are specified in calendar time, which represents the time elapsed
since the Epoch (00:00:00 on January 1, 1970, Coordinated Universal Time (UTC)).

Declaration
Following is the declaration for difftime() function.
double difftime(time_t time1, time_t time2)

Parameters
time1 − This is the time_t object for end time.
time2 − This is the time_t object for start time.

Return Value
This function returns the difference of two times (time1 - time2) as a double value.


C function time()
https://www.tutorialspoint.com/c_standard_library/c_function_time.htm

Description
The C library function time_t time(time_t *seconds) returns the time
since the Epoch (00:00:00 UTC, January 1, 1970), measured in seconds.
If seconds is not NULL, the return value is also stored in variable seconds.

Declaration
Following is the declaration for time() function.
time_t time(time_t *t)

Parameters
seconds − This is the pointer to an object of type time_t,
where the seconds value will be stored.

Return Value
The current calendar time as a time_t object.


    You can find all my C programs at Dragan Milicev's pastebin:

    https://pastebin.com/u/dmilicev

*/

#include <stdio.h>
#include <time.h>       // for time(), difftime()
#include <stdlib.h>     // for exit()

int main(void)
{
    int i, j;
    time_t seconds;
    time_t start_t, end_t;
    time_t now;
    time_t current_time;
    double diff_t;
    struct tm *ts;
    char       buf[80];
    char* c_time_string;

    printf("Starting of the program...\n");
    time(&start_t);     // get start time

    printf("\n Work something ... \n");
    for(i=0;i<100;i++)
        for(j=0;j<100;j++)
            printf(" %6d ", i*j);

    time(&end_t);       // get end time
    diff_t = difftime(end_t, start_t);

    printf("\n Execution time = %f \n", diff_t);


    seconds = time(NULL);
    printf("\n Hours since January 1, 1970 = %ld \n", seconds/3600);


    now = time(NULL);   // Get the current time

    // Format and print the time, "\n ddd yyyy-mm-dd hh:mm:ss zzz \n"
    ts = localtime(&now);
    strftime(buf, sizeof(buf), "\n %a %Y-%m-%d %H:%M:%S %Z \n", ts);
    puts(buf);


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

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

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

    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 \n", c_time_string);

    exit(EXIT_SUCCESS);


    return 0;

} // main()