C&S personal calendar conflicts with group calendar

A person in our company was away on holidays yet in the group calendar it showed that she was available. Thinking that maybe she was back I looked at her personal calendar and there it showed her as away on vacation. Anyone know why this might occur and how to fix it?

Subject: C&S personal calendar conflicts with group calendar

Try rebuilding your busytime.nsf. (Shut down your server, delete busytime.nsf, and restart. The database will rebuild on startup). There are many instances here on this forum of busytime corruption.