The Time calculator or time subtractor calculator is a free online calculator that can be used to “add” or “subtract” two-time values. The input fields can be left blank, and this will be assumed to be 0 by default. Duration of Time: The duration of an activity is the difference between the starting time and finishing time. When the starting and finishing time is given, we can easily find the duration of any activity. For example, if Terry boarded her school bus at 7 o’clock and she reached school at 8 o’clock, the duration of her travel is 1 hour.