End date/time before start date/time – cannot delete item

We recently upgraded from Outlook 2003 to 2007. I have SP2 for Office 2007 insalled on Windows XP computers (with SP3). The Exchange 2003 Server is also fully patched. Users get very similar errors such as ‘cannot delete meeting – end time is before start time’ and other similar error messages. Business day is set to start at 8 am and end at 5 pm.