why does my calendar app keep reverting events to the first version?

MacBook Pro late 2012- os10.8.5

Whenever I try to update an event in the calendar app it does not stick on my Mac book Pro. It goes right back to the way it was and it's so infuriating to have to do everything twice on my laptop. Calendar seems to be working fine on my iPhone. Can anybody tell me what is going on? Thanks for any information or advice.