Here's how I'd do it. There are probably lots of other ways. Maybe some of them are better but this should work.
// mktime format: hour minute second month day year
$publish = mktime(0, 0, 0, 5, 13, 2006);
$now = time();
if ($now > $publish) { echo "extra content"; }
time() is a ten digit number which represents the number of seconds since Midnight Dec 31, 1969. mktime() converts day, month and year to that same format.
You could easily have a stop publish time too like this:
$publish = mktime(0, 0, 0, 5, 13, 2006);
$stop_publish = mktime(0, 0, 0, 6, 13, 2006);
$now = time();
if (($now > $publish) and ($now < $stop_publish)) { echo "extra content"; }
This way, your content would only be up for a month.
