As wee eddie has said, since time doesn't have a negative concept formatting as TIME won't work.
Is this workaround acceptable (formatted as time):-
A1=12:00 B1=14:00 C1=If(b1>a1,"-","+") d1=If(b1>a1,b1-a1,a1-b1)
This gives an extra cell (centred if necessary) with the required 'minus/plus' sign. You could also use Conditional formatting on cells C1 & D1 to show red i.e C1 condition "Cell value is equal to "-" and set font to red. D1 condition is "Formula is B1>a1" and set font to red.
Hope that's of help.