Oh, the Now() threw me.
I created two textboxes. The control source for the first was
=DateDiff("n",[testDate1],[testDate2])
This changes the difference to minutes.
The control source of the second is
=[TotalMin]\60 & Format([TotalMin] Mod 60,"\:00")
This formats it the way you want.
So you can make the first textbox invisible.