The above fomulae (Bob's and mine) only work for hourly times as in the
example given. But for other times an adjustment is needed eg 2115 to
800 should give 1045 (not 1085). This should work for all times entered
in "hhmm" format:

=--TEXT(MOD(TEXT(B1,"0\:00")-TEXT(A1,"0\:00"),1),"hhmm")