If a custom profile field of type 'date' is defined,
a) it is not possible to reverse from 'default to today' once selected and saved
b) whatever the default is, the selectable date range goes from TODAY to exactly 100 years ago - so one has no way to select a date in the future.
For b), I found a solution by hacking includes/functions_profile_fields.php , in line 669 the code has to be patched to allow a broader selection of the year.