Time difference calculation using hidden value

How do i calculate time? i.e if i have 2 question one is asking the start time and the other is asking end time. How do i calculate the difference in hours?

here you go!

https://confluence.dimagi.com/display/commcarepublic/Calculations+with+Dates+and+Times#CalculationswithDatesandTimes-Calculationsusingtimequestiontypes