I have opened an existing excel file. When I opened it all the cells were presented in date format instead of general. This has happened several times now even when I change all the cells back in general format. Has anyone an idea why excel change the number format of the cells?

Any macros in the workbook?
If it's not full of sensitive information could you attach the offending workbook?

It's an occasional bug in Excel where the number format for the Normal cell style changes. It often relates to having a lot of pivot tables in the file.

There are macro's in the file.
The change of the number format doesn't take place every time and with more files.
I guess it has nothing to do with the file but more with excel and/or some settings.