[nhawk]

You're right, a little brain freeze here...

should be this to account for 00:00:00 time..

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

Because dateline will be greater than or equal to unix timestamp of '2012-02-22 00:00:00'.

And dateline will be less than a unix timestamp of '2012-02-23 00:00:00'.