Although this is a bit late : If he uses timestamps then, in my experience he will run into trouble if he tries to perform any MySQL Date arithmetic / calculations on the timestamps - and the over head to do the same in PHP has the potential to become very expensive as it would involve selecting ALL records and then performing comparisons / calculations on the converted dates.
And I concur with jasonbar - PHP is looking at the delimiters of the date and considers it to be a US format date! He will need to run a str_replace('/','-',$_POST['date']) BEFORE using the date() function.
So, to fix this on an incoming request:
$mysqldate = date('Y-m-d', str_replace('/','-',strtotime($_POST['date'])));
So long as the data type for the target column is datetime anyways!