More grief over Hours v TIME- Negative hours

My staff timetable is coming on a treat, but once again the TIME formatting/formulas are proving illogical (to my mind at least!)

I have a very simple subtraction task I thought....

Cell AA14 1265:00
AA15 1280:36

In AA16 is the formula for subracting AA15 from AA14.
All is fine until I get negative numbers (all cells are formatted to [h]:mm and I'm using 1904 time options
Bog standard AA14-AA15 gives nonsense (-15:36) unless its a normal positive number.

Any help would be geartly appreciated
Thanks

actually the answer is correct... im think if that time has a day month and year??? try to add that...^_^

Doh,
Can't believe it. I've been going round in circles and missed the answer staring me in the face.
Thank you Greggy