We are arriving in Rome early on a Saturday morning. We only have Saturday and Sunday in Rome for sightseeing. We plan to go store our luggage and go straight to the Vatican Museum-then on to St Peters. I wanted to pre book the tickets to skip the line and see they require you to set a time. I didn't read anywhere what happens if you miss your time and arrive later. My concern would be what if our flights late etc and we don't make it by the reservation time. Any advice about this?