Calculating number of hours between 2 start and finish times on su

Hi all,

I have a sub form that has 'VisitStartTime' and 'VisitFinishTime' fields 
that allows the user to enter times in date format. I also have another
field 
I inserted called 'HoursComplete'. My query is, how do I enter a formula
for 
the 'HoursComplete' field that calculates and displays the hours and
minutes 
between the 'VisitStartTime' and 'VisitFinishTime' fields?

Many Thanks

Ammo