Instead of saying the date only or "unspecified", when a user has a birthdate with no year in it, a white box seems to be shown instead of the table column.
admin/users.php?adminsid=youradminsid&action=stats&uid=youruid
Edit: This is fixed in 1.2.8 only since there will be a new Admin CP in 1.4
This bug has been fixed in the latest code.
Please note the latest code is not live on the site or for download. An update will be released which contains this fix.
In admin/users.php find on line 1167 find:
$membdayage = '';
and replace with
$membdayage = $lang->not_specified;