simplified version -- will count the number of unique posters (assumes you have mysql 5)

Code
SELECT count(posters.POST_POSTER_NAME) from (SELECT Count(up.POST_ID) AS UserPosts, up.POST_POSTER_NAME
FROM ubbt_POSTS up
WHERE up.POST_POSTED_TIME > UNIX_TIMESTAMP( '2008-04-20 00:00:00' ) -86400 *60
GROUP BY up.POST_POSTER_NAME) posters

many other ways to 'skin a cat', but this works too wink