MYSQL QUERY NOT SAVING DATE OF BIRTH

MYSQL QUERY NOT SAVING DATE OF BIRTH

Mysql query is not saving the date of birth as expected but only saving it
as 0000-00-00 from a php registration form, what might be the matter?
ThanK You
<?php
if(isset($_POST['registration']))
{
require "connection.php";
$FirstName = strip_tags($_POST['FirstName']);
$LastName = strip_tags($_POST['LastName']);
$Phone = $_POST['Phone'];
$Month = $_POST['month'];
$Day = $_POST['day'];
$Year = $_POST['year'];
$Date = $Year."-".$Month."-".$Day;
mysql_query("INSERT INTO users (FirstName,LastName,Phone,DOB) VALUES ( '"
. $FirstName . "','". $LastName . "','" . $Phone . "','" . $Year . "-" .
$Month . "-" . $Day . "')") or die("".mysql_error());
echo "Successful Registration!";
}
?>