Hello,
I opened the file on my computer and yes, all cells were setted to a date format. I then changed all cells to the general format. The format remains if I close and open the file again. However, now all cells are in the general format and therefore you may verify all formats. I attached this file.
I noticed that when I add a new sheet, all cells are set to a date format. This is due to a modified/incorrect assigment of a format style. Also, you have many "Normal" formats in your styles definitions for the existing sheets. You can see them, if you open the file and go to the home tab. In the styles group, "Normal 1", "Normal 2", and so on are listed. In a new Excel file, normally "Normal", "Bad", "Good", etc are visible.
These styles can be edited. For this right click on one entry and open the style settings by clicking on "Modify". Then, in the dialogue, click on the button "Format". The format cells dialogue appears and you see that the date format is selected instead of the general format.
If you add a new sheet to your workbook, then you have a default style which is applied. Now edit the "Normal" style. The default setting is "Date". Change this to "General" and confirm your changes with Ok. Then, when adding new sheets, the default format is now "General".
I did not test out, if changing/deleting all the duplicate styles from you sheets will change the behaviour for your file. But I think, these styles are the problem.
Best,
Mourad
