Comparing two time values

Hi, I would like to restrain field staff from accidentaly typing a time
value that is higher than or lower than a previous time value entered.
For example if a client arrived at 7.30AM EAT and is being registered at
7.30AM EAT or latter say 9.00AM EAT, the registration time should not be
less than 7.30AM but can be equal to or greater than 7.30AM EAT. Lkewise
the arrival time should not be greater than the registration time.