[Logician]

PHP Code:

$ago= mktime (date("H"), date("i"), date("s"), date("m")-30, date("d"), date("Y"));
$countrecentposts=$DB_site->query_first('SELECT COUNT(*) AS 30recent FROM post WHERE dateline >= $ago'); 
$total30recent=$countrecentposts['30recent']; 


If your DB size is large, this query can be a performance killer so if this is the case add an "index" to your dateline field in post table.