I don't recall a format changing for a cell containing a date (not a formula to
another cell with a date) when using the same pc.

But I like mm/dd/yyyy (4 digit years in my dates).  If I change my short date
format in windows (under regional settings in the control panel) and I format my
date as m/d/y, excel will use my windows setting.

If I open that file on another pc that has a windows short date of (say) m/d/yy,
then the date formats differently--to match the user's windows preference.

Could that explain it?

If no, you may want to post a little more info.