Wait time on Shipment Stop

I have a requirement which should calculate the wait time on shipment stops through tracking events entered.

I have entered the Pickup Appointment time on stop 1 and Delivery Appointment time on stop 2. The wait time is getting is calculated only if Appointment Pickup/Delivery dates/times is greater than Actual Pickup/Delivered dates/times. By logic it should be vice versa. Can anybody share their experience on this subject.