A problem was recently discovered with the Office 2007 Compatibility Pack file converters when used to open Office 2007 Open XML files in previous Office versions, when those files have custom Date/Time properties attached associated with SharePoint. If the documents are opened and saved back to SharePoint in Office XP/2003, the Date/Time property may accidently get modified to an incorrect time due to a bug in UTC-to-Local time conversion.

See the following post for full details:
SharePoint Document Library Date Time Columns May Be Incorrectly Changed for Office 2007 Documents if Opened and Saved by Office XP/2003