Change the value in the user join date field
Old users:  update the users table in your database.  It will be far more efficient.

New users:  https://api.drupal.org/api/drupal/modules!user!user.api.php/function/hook_user_presave/7     I don't think the date/time are saved in that form, so what you could do is find the UID and update the table using a sql query after the save is done.