Excel File Changed Cell Format on its Own?

[Jon Melone]

Hi,
I have a workbook with many sheets. Yesterday the columns were formatted as Number and when I opened the workbook this morning, all columns in all sheets are now formatted as Date. No other user has access to the file. It seems my only option is to reformat the cells back to Number manually. Has anyone heard of this happening before? I sure would like to prevent future occurrences.

Thank you,
Jon