I've had this problem for quite a while now. Virtual Center seems to periodically drop performance statistics. I would show another example where it dropped several days, but when I installed the upgrade yesterday I apparently selected a bad option along the way and reinitialized my database.
NTP is configured correctly and my Hosts and VC are synced to the same NTP source. When I examine the clocks they are to all appearances exactly in sync. The timezones are also in sync, and other databases on the SQL Server holding the data don't drop records w/o a good reason. Tech support has taken a couple stabs at this w/o much success. Any ideas?