A common issue with the built-in date formats. But month and day without the year will not always provide a valid date. Feb 29 is a valid date almost 25 of the time..
You can convert the date to the JavaScript date object and then create a variable with the desired format and compare the entered value to the formatted variable, but you would need to know the year the month and day are part of. I would use the full year or you could encounter issues with dates prior to January 1, 1950, sort of like the millennium bug