The starting date is a text field with a creation of a Timestamp (Date and Time).
Same for the ending date.
That's a mistake there. Make the two fields, fields of type timestamp, not text.
Then EndingTimestamp - StartingTimeStamp will give you the elapsed time in seconds. You can specify a time result type to display the result as HH::MM::SS or you can use a number result type but divide by 60 to get elapsed time in minutes or by 3600 to get elapsed time in hours.