time selection online ordering

We noticed lately that a customer can place an online order at 6:24pm to be picked up at 6:30pm. Because that is only 6 minutes that should not be allowed. The ‘rolodex’ clock should therefore default to ‘6:45 pm’ and allow us time to prepare the meal. There has to be a way to set your behind the scenes programming for the clock that when the time they are ordering is less than 15 minutes the clock should not allow them to order unless they select a later time frame.