Excel automatically converts any numbers to a date

Hey guys,

Here's the lowdown.

An excel sheet, originally created in Excel 2003 is saved in .xlsx format in Excel 2007.
All numbers in the document changes to date format when the document is reopened.
The user then changes the format of all numbers (which is painstaking to say the least) and saves the document again. When reopened, guess what, its back to date format.

This issue does not occur with new documents or old documents being saved in .xls format in Excel 2007.

Any clues as to what's going on here?