Drupal schema API not supporting MySQL date data type?
You could define it yourself, have a look at http://drupal.org/node/159605#comment-1674678.

You should however only do this if you are sure that you will only ever use/support MySQL.

Portability with date data types among different databases is almost impossible. Drupal 7 initially had date types defined but they were removed again because there is simply no way to make them work consistently across multiple database types.

You might want to look at http://drupal.org/project/date, I'm pretty sure that allows to use dates that can not be mapped to a UNIX timestamp.