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In this example, you will learn to calculate the difference between two time periods using a user-defined function.

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To understand this example, you should have the knowledge of the following C programming topics:

• C User-defined functions
• C struct
• C Structure and Function
• C structs and Pointers

---

Calculate Difference Between Two Time Periods

#include 
struct TIME {
   int seconds;
   int minutes;
   int hours;
};

void differenceBetweenTimePeriod(struct TIME t1,
                                 struct TIME t2,
                                 struct TIME *diff);

int main() {
   struct TIME startTime, stopTime, diff;

   printf("Enter the start time. \n");
   printf("Enter hours, minutes and seconds: ");
   scanf("%d %d %d", &startTime.hours,
         &startTime.minutes,
         &startTime.seconds);

   printf("Enter the stop time. \n");
   printf("Enter hours, minutes and seconds: ");
   scanf("%d %d %d", &stopTime.hours,
         &stopTime.minutes,
         &stopTime.seconds);

   // Difference between start and stop time
   differenceBetweenTimePeriod(startTime, stopTime, &diff);
   printf("\nTime Difference: %d:%d:%d - ", startTime.hours,
          startTime.minutes,
          startTime.seconds);
   printf("%d:%d:%d ", stopTime.hours,
          stopTime.minutes,
          stopTime.seconds);
   printf("= %d:%d:%d\n", diff.hours,
          diff.minutes,
          diff.seconds);
   return 0;
}

// Computes difference between time periods
void differenceBetweenTimePeriod(struct TIME start,
                                 struct TIME stop,
                                 struct TIME *diff) {
   while (stop.seconds > start.seconds) {
      --start.minutes;
      start.seconds += 60;
   }
   diff->seconds = start.seconds - stop.seconds;
   while (stop.minutes > start.minutes) {
      --start.hours;
      start.minutes += 60;
   }
   diff->minutes = start.minutes - stop.minutes;
   diff->hours = start.hours - stop.hours;
}

Output

Enter the start time.
Enter hours, minutes and seconds: 12
34
55
Enter the stop time.
Enter hours, minutes and seconds: 8
12
15

Time Difference: 12:34:55 - 8:12:15 = 4:22:40

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In this program, the user is asked to enter two time periods and these two periods are stored in structure variables startTime and stopTime respectively.

Then, the function differenceBetweenTimePeriod() calculates the difference between the time periods. The result is displayed from the main() function without returning it (using call by reference technique).