Column format not adhered to

I have the format of two columns of a spreadsheet set to “Number General” and up to now it has been displaying correctly.
However, when I opened the spreadsheet today, it was displaying both columns in date format (e.g. instead of “97” it shows “06/04/00” and the tooltip shows the correct value of “97”).
When I open the “format cells” dialog, the highlighted category and format is still “Numbers/General” as I would have originally set it.
It appears that the only way to correct this was to set the format to something else and then reset it back to the correct category and format.
Does anyone have any idea what could have caused this?

Please attach a sample file if you still have it. Also, do not post as community wiki. See guidelines for asking.

If it is a number, then why not format as Number -1235 (format code 0) for example? That would fix the issue for sure. General may do any number of things. Having said that, without more information, I do not know why the problem occurred.

No sample available - as I indicated, the format was still showing as 'Numbers/General" and I reset it to something else before resetting it to “Numbers/General” again, and I didn’t think to save the earlier version.

and - - Sorry!! I didn’t intentionally check the “community wiki” checkbox (can it be unchecked?).

“Can it be unchecked” - it doesn’t look like it. That leaves either posting the question again or else simply remembering not to check the box for future questions.

If number format is General and a date is displayed it may be that there is conditional formatting applied which style includes a date format. Check menu Format → Conditional Formatting → Manage…

The only conditional formatting was related to background colour.