Born after 1/1/3100 means...
* You were born 3/5/3120
* You were born 10/2/3101
* You were born 1/1/3200

You could select any birthdate that isn't earlier than 1/1/3100.

So then we can make today's date 6/21/3150 or something. As long as it's in that general century, it doesn't matter a whole lot.