A mailbox had been switch to Exchange 2007, a problem occurs when a delegate to the mailbox opens the mailbox's calendar then clicks on the 'Calendar...' button to re-plan the meeting. A window appears to fix the date and the calendar bar displays the date of August 31 4500. Numerous things have been tried such as cleanfreebusy (Microsoft KB Article 296192 - OL2002: Additional Command-Line Switches - http://support.microsoft.com/kb/296192) on the mailbox but it didn't fix the problem. The delegate doesn't have the problem with other delegated mailboxes.
This is a know Microsoft problem related to Exchange Server 2007 Enterprise Edition and Exchange Server 2007 Standard Edition.
Please see Microsoft KB article 930801 - The calendar opens to August 31, 4500 or to September 1, 4500 when you view a meeting request in Outlook - http://support.microsoft.com/kb/930801