Inserte la fecha de nacimiento en la base de datos utilizando PHP

<!DOCTYPE html>
<html>
<head>
   <title></title>
</head>
<body>
<form method="POST" action="">
<select name="Year">
  <option value="">Select year</option>
  <?php for ($i = 1980; $i < date('Y'); $i++) : ?>
  <option value="<?php echo $i; ?>"><?php echo $i; ?></option>
  <?php endfor; ?>
</select>
<select name="Month">
  <option value="">Select month</option>
  <?php for ($i = 1; $i <= 12; $i++) : ?>
  <option value="<?php echo ($i < 10) ? '0'.$i : $i; ?>"><?php echo $i; ?></option>
  <?php endfor; ?>
</select>
<select name="Date">
  <option value="">Select date</option>
  <?php for ($i = 1; $i <= 31; $i++) : ?>
  <option value="<?php echo ($i < 10) ? '0'.$i : $i; ?>"><?php echo $i; ?></option>
  <?php endfor; ?>
</select>
   <input type="submit" name="sub">
</form>
</body>
</html>
<?php
$conn = mysqli_connect("localhost","root","","getdob");
if(isset($_POST['sub']))
{
$Year = $_POST['Year'];
$Month = $_POST['Month'];
$Date = $_POST['Date'];
if ($Year != '' && $Month != '' && $Date != '') 
   {
   $date = $Year.'-'.$Month.'-'.$Date;
   $sql="INSERT INTO dob VALUES  (Null,'$date')";
   if (mysqli_query($conn,$sql))
   {
      echo "Record added!";
   }
   else
   {
      echo "Error!!!";
   }
   }
else
   {
      echo "Please Select Day, Month and Year!!!";
   }   
}
?>
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