The "time.h" header file in C programming provides functions for working with date and time values. Some of the commonly used functions in "time.h" include:

1. "time": Returns the current calendar time as the number of seconds since the Epoch (January 1, 1970).

2. "gmtime": Converts a time value to a broken-down time structure in Coordinated Universal Time (UTC).

3. "localtime": Converts a time value to a broken-down time structure in the local time zone.

4. "asctime": Converts a broken-down time structure to a string in a format like "Sun Sep 16 01:03:52 2018\n".

5. "ctime": Converts a time value to a string in a format like "Sun Sep 16 01:03:52 2018\n".

6. "difftime": Computes the difference between two time values in seconds.

7. "mktime": Converts a broken-down time structure to a time value.

Here is an example of using the "time.h" header file in a C program:

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

int main() {
    time_t t = time(NULL);

    // print the current calendar time
    printf("Current time: %s", ctime(&t));

    // convert the current time to UTC
    struct tm *utc_time = gmtime(&t);
    printf("UTC time: %s", asctime(utc_time));

    // convert the current time to the local time zone
    struct tm *local_time = localtime(&t);
    printf("Local time: %s", asctime(local_time));

    // compute the difference between two times
    time_t t2 = time(NULL);
    double diff = difftime(t2, t);
    printf("Difference in seconds: %f\n", diff);

    // convert a broken-down time structure to a time value
    struct tm tstruct = {
        .tm_sec = 30,
        .tm_min = 10,
        .tm_hour = 5,
        .tm_mday = 10,
        .tm_mon = 8,
        .tm_year = 121,
        .tm_wday = 3,
        .tm_yday = 0,
        .tm_isdst = -1
    };
    time_t t3 = mktime(&tstruct);
    printf("Time value for 10:10:30 AM on September 10, 2021: %ld\n", (long)t3);

    return 0;
}

In this example, the "time.h" header file functions are used to work with date and time values. The "time" function is used to get the current calendar time as a time value. The "gmtime" and "localtime" functions are used to convert the current time to a broken-down time structure in Coordinated Universal Time (UTC) and the local time zone, respectively. The "asctime" and "ctime" functions are used to convert a time value and a broken-down time structure to strings. The "difftime" function is used to compute the difference between two time values. Finally, the "mktime" function is used to convert a broken-down time structure to a time value. The output of the program will vary based on the current time, but an example output could be:

Current time: Sun Feb 27 23:44:33 2022

UTC time: Mon Feb 28 04:44:33 2022

Local time: Sun Feb 27 23:44:33 2022

Difference in seconds: 17.000000

Time value for 10:10: