This page from the PHP manual will explain everything you need to know about how to format the date properly: http://php.net/manual/en/function.date.php
Do you know that the DATE function uses your servers timezone settings? This means your date could be wrong compared to your actual location. If that is an issue look into the gmdate function here:
http://www.php.net/manual/en/function.gmdate.php
Your code seems right though for the date, it should be working. Go back and add quotes around your POST though.
You have this -> $_POST[check]
It should be this -> $_POST['check'] OR $_POST["check"] (First one is the proper way)