I'm new to functions and formulas - how do I create one to automatically
update a cell so that it shows the difference between two cells that display
times.

For example, cell A1 contains 2:30pm and A2 contains 2:45pm - I want A3 to
display the difference, regardless of whether it is a negative or postivie.
In fact, I would prefer that it made no reference to whether it's a negative
or not.