We have a workbook that is spontaneously formatting every cell in every sheet to date format. It is a macro enabled workbook.
I have gone through the workbook sheet by sheet and corrected the formatting. Saved the changes.

No matter what, the date format problem reappears again.

We cannot find anything we have done to cause the problem. The date format shows up on sheets we haven't used recently.

Bizarre.

Note: It takes at least a minute to open the workbook (4mb). Saves are 4 or 5 seconds.