You should register for a shuttle that is at least two hours before your scheduled flight time. The drive to the airport is usually 35-40 minutes depending on traffic. So a shuttle scheduled to leave at 2pm will arrive at the airport around 2:35-2:40.
The time indicated is the time the shuttle will leave the airport. If your flight gets in after the shuttle time, or close to that time (less than 15 minutes before), please select a later shuttle. For example, the 12pm shuttle will be leaving Dayton Airport at 12pm and a flight arrival time of 11:55am should register for the 3pm shuttle.