The previous tip submitted for the Freetime corruption fix should keep everyone's freetime up to date. However, if a user reports during the day that a calender is showing all busy time when it shouldn't, then you can update a single user with the following.
Go onto the Notes client on the server and open the database "Local free Time Info on Local"
Find the user concerned and delete the entry then press F9 to refresh.
Go to the Lotus Domino Server window and type TELL SCHED VALIDATE and press enter.
After a couple of minutes the user's free time will be recreated.
Obviously you can delete a few users or even all of them and recreate immediately without having to wait for a scheduled run overnight.