if you have dynamic dropdowns extension, you can use 2 recordsets to create a dynamic array, so that options for the second dropdown depend on the choice made in the first.
testdd.php
here, choosing 6.00 or 6.30 as the first option ensures that the second option contains only items at least 2 hours later, up to 4 hours later
choosing 7.00 as the first option reduces options in the second dropdown to 2-3 hours later