O yes, it gets resolved by this format. Just did it. the issue is that the Upwork team should work on showing the require tooltip when the wrong DOB is being entered. they show only when a correct is entered. Please write in this format to avoid any inconvenience.
Could you try clearing your cache and cookies and then adding your DOB. The correct format is YYYY-MM-DD, if you`re still experiencing the same problem after this let me know. Thank you.