I dont know any link, but ur code can be something like this. Please check the links, the functions are self-explanatory.
$lastRecDay = "2013-01-11"; // this will be coming from ur db
// create a date object
$date = new DateTime($lastRecDay);
//add 20 days to it
$date->add(new DateInterval('P10D')); // ur date is 20 from last day
//its 20 days+ since the posting
if( $date->format('U') <= strtotime('now'))
{
// your code re-open goes here
}