If you go into Entourage and right click on your calendar in the left hand column, click "Folder Properties" then on the window that comes up click "Empty" under "Empty Cache". This will delete all of your calendar entries from Entourage and re-download them from the server. As long as your Mac's clock is correct, all the entries should be fine. Once you hit the empty cache button it will take probably 15-20 minutes to completely re-populate your calendar from the Exchange server.