Tickets are not a reservation – they are strictly a boarding pass. If you are using your tickets before they expire, there is no need to return/exchange them even if there is a different time noted on the ticket – just show your boarding pass to the driver when you are ready to travel. As a reminder, one-way tickets are valid for 90 days, and round-trip tickets are valid for 180 days, which means you may travel any day or time until the expiration date.