I did some manual join date modification for some of the oldest users on a forum when I migrated. Like Jeff suggested, you basically need to pick out the users manually, like so:
user = User.find_by(username: 'krilnon')
user.created_at = Time.now
# user.created_at = Time.parse('2013-02-04 3:00 PM') # to backdate
user.save!