Drupal stores dates in Unix time format. I had to look this up because I was trying to find email addresses for users who joined after a certain date. I used a SQL query in phpmyadmin and then exported into an OpenOffice spreadsheet:
SELECT `mail`, FROM_UNIXTIME(created) FROM `users` WHERE created > 1199145600 LIMIT 500;
That returns 500 emails from users who registered since January 1, 2008. The
FROM_UNIXTIME(created) converts the Unix formatted time into something human-readable.
This Unix time converter is useful for turning a human-readable date into a Drupal date and vice versa.
See also this post.