Excel Date & Time

Hey all,

I’m trying to come up with a formula that calculates the departure time, return time then time remaining until return (ETA). Example:

Cell A1 (Scheduled Departure Time)
Cell A3 (Scheduled Return Time)
Cell A5 (ETA – Time Remaining until arrival)

How do I do setup this for real time tracking? There are 49 vehicles that operate 24/7 in my fleet. Being able to do this would be a TREMENDOUS help in scheduling. Please help?!