Excel Format Changes on its own

I have a substantial sized workbook with just under 200 sheets,(total of 3821KB). Upon opening the file I noticed that any cell with a formula or link had changed to a time format since the last time I opened the file. The formula/link is there, but instead of the correct info showing it displays 12:00:00AM. If I reformat the cell it does display the correct result.

I did have simular issues in excel 2003 with another file, but I believe that was just too much formatting which make me suspicious that this may be related to size or limitations. My understanding is that in 2007 formatting is unlimited.

Any susggestions as to why cell formats have changed or how to get them all back without reformatting every individual cell?

Thanks for your help.

If you open the list of custom formats, you may see one that looks like:
"[$-409]d-mmm-yyyy h:mm AM/PM;@"

If you delete it then all the affected cellls should revert to general format.

I've seen this reported on other forums without yet seeing a reason or 'proper' fix.
Regards
Mike