(Using Excel 2002)

I have created a staff timesheet which automatically calculates hours worked
etc. etc. The one thing I can't work out is how to use data validation using
time. In one cell I have the time the person went on lunch, in the next cell
I have the time someone comes back from lunch. I want to make sure that the
second value is at least 30 minutes later (i.e. they have to take at least 30
minutes for lunch). How do I achieve this? (In all cells I am using [h]:mm
format for time)