Correct format for date field in Add listing

Hi,

I wanted to add a “year founded” field to our custom post type form but
I don’t understand what it’s looking for under “Date format”. I clicked on [valid date formats] but I can’t seem to insert the right info in this field to save the form. What is it looking for? actual date example or mm/dd/yyyy?

I would like to be able to have the user just add month and year but it doesn’t look like that’s an option.

Thanks,