@rich20 sorry if there was a misunderstood, the intention of my comment is to help quickly identify error background and all related commits to fix the problem, is not for close this issue. I agree with this new issue.
No problem @sshcli
The problem seems the script of the calendar. It's not possible in the frontend manually to enter this date of birth 01/01/0001. Try it, the date will cutted to 01/01/1 and after save it is changed to 01/01/2001
This solution would only work if you can choose the year on the datetimepicker. When the default value is 01/01/0001 you need currently 1999 x 12 clicks if your birth date is in the year 01/01/2000. You can go for- or backwards only per month.
Sorry, was nonsense what I've written.