How to Find which one is greater between 2 times

Hi All,
I am trying to use If condition to find which one is the max time however i am not able to do it.Below is the example



"Login Time
IST" Actual Login Time Discrepancy
23:45 23:53
18:15 18:35
18:15 18:13
19:15 19:14
18:15 18:15

In discrepancy i need to check if actual login time is more time than login time IST then it should subtract actual login time from login time IST or else it should display login time IST .

Note: Actual Login Time is in Time format.