Default Date Input Format.... Shared files

I know this question comes up all the time... but I have a new spin.  Changing the computer region settings and the account region settings are not fixing this problem.  We are using a shared Excel file and it isn't working for anybody else (we are an international group)... I've tried changing their settings on their computer or tablet, and tried changing the settings in their account.... nothing.  I built the file on a computer with mm/dd/yy regional settings, but with an account with dd/mm/yy settings.   No matter what I change, or where I change it, the default input date for everybody using the file is mm/dd/yy…. There must be some setting in the file somewhere that is effecting this. Please advise