How do I calculate charges based on elasped time & rate?

I want the time spent on a service to display in HH:MM

Then I want to multiple this elasped time by a rate, say $25/hour. I can
put this rate on the form and keep this cell outside the print area, or I can
type "25" into the fomula, either will work for me.

When I try to input a custom format, as described in help to be <h:mm
(where < are square brackets), the data input shows in the input field as
1:30 AM, even after deleting the "AM" part, so it refuses to treat this as
elasped time.

Please HELP!!!! I don't want to have to firgure the fractions of an hour
for each task in order to perform this calculation.

Thanks in advance