Calendar events changing time after I set them at the correct time

This is not happening with every event I make, but it has happened to a few of them on this week's schedule. One of them I have set from 4:30 - 5:30, but it keeps jumping to 4-5. I reset it to 4:30-5:30 and then 2 minutes later it's jumped back up. How do I keep it at the correct time slot? I have already checked my time zone settings.