it is more useful to use the time() function so you can do more with the date you enter.
<input type="hidden" name="postdate" value="<?php echo time();?>">
then you can echo it with your sql query.
echo date("Y-m-d",$row["postdate"]);
You can query dates in a range of your choice.
$fromdate = time()-(604800); // 7 days back
$todate = time()-(1209600) // 14 days back
$sql = "SELECT * FROM table WHERE postdate BETWEEN '$fromdate' AND '$todate' ";