[kh99]

Try this:

SELECT userid, username, COUNT(*) as count FROM post 
WHERE visible=1 AND 
UNIX_TIMESTAMP('2012-02-22') < dateline AND
UNIX_TIMESTAMP('2012-02-23') >= dateline
GROUP BY userid ORDER BY count DESC

I tried to come up with one that let you just enter the date once, but I'm not that familiar with the mysql functions. On the plus side, this lets you find posts between any two dates.