Date fields must be valid according to your system setting. If you establish your Regional Settings so you use YYYY/MM/DD then you will need to do two things with your existing data and file:
1) Create an empty clone of this file under the new OS settings. In that way it will inherit the new YYYY/MM/DD format.
2) Export your data and import it into the new clone.
After doing some research, I thought that you could fix this system settings bug by going to File->File Options->Text-> Checking "Always use file's saved settings"
Is that not a fix?
And what are the 'files saved settings?' They are settings in place at the moment the file is created and they are planted within the schema and can not be changed within a file. The only way to use the new OS settings is to 'create' a new file (or clone an existing one).