Drupal stores dates in Unix time or timestamp format, so here is the code to show it in MySql format:
SELECT FROM_UNIXTIME( created ) FROM `users`
The function FROM_UNIXTIME, will convert the format for you, here is a simple query sample to get a list of users created today:
SELECT `mail`, FROM_UNIXTIME(created) FROM `users`
WHERE FROM_UNIXTIME(created,'%d-%m-%Y') = '21-01-2011'
Well, you can change the date, of course!