Time Difference When Past Midnight?

I have a field that works out the difference in time between two fields. However, whenever the end time goes past midnight, it calculates the difference as a minus figure. I understand that this is because of the date issue, but I cannot seem to find a way around it.

Here is what I have so far:

time_of_referral field. This is the start time. It is a short time field that is filled in by the end user.

time_of_arrival field. This is the end time. It is a short time field that is filled in by the end user.

Text31 field. This is not visible to the end user, and has the following control source =DateDiff("n",[time_of_referral],[time_of_arrival])

Text33 field. This is an unbound text box with the following control source =[text31]60 & Format([text31] Mod 60,":00")

This works out the time difference and presents it as hours and minutes.

I have seen solutions that add a day to the end time, but unfortunately this does not work when the end time DOESN'T go past midnight. Some will go past midnight, some won't.