Calculating Time between two 'Time-of-day' values - TIA Portal

Objective is to find out the time (Hours, Minutes and Seconds) between two given time of day values. We need a positive value of the time and it should always calculate the time between start and end times in positive (clockwise) direction. Example 1 - If start time is 13h:00m:00s and end time is 15h:00m:00s then the time value should be +2 hours. It should not be -2 hours. Let's see how to program it in TIA Portal