MYSQL Insert using Dropdowns and Session Variables

I've been trying to solve this problem for a few hours and can't seem to make headway. I am creating a booking form and it involves 2 dropdown menus and the use of some session variables. HTML

<form accept-charset="UTF-8" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"])?>" method="POST">
    <input type="date" data-role="date" name = "Date"data-inline="true" id ="Date" placeholder="Click here to select a date">

   <br>
    <select id="Time" name="Time">
        <option value="Null">Select a Time</option>
        <option value="9am">09:00</option>
        <option value="9.20am">09:20</option>
        <option value="9.40am">09:40</option>
        <option value="10am">10:00</option>
        <option value="10.20am">10:20</option>
        <option value="10:40am">10:40</option>
        <option value="11am">11:00</option>
        <option value="11:20am">11:20</option>
        <option value="11:40am">11:40</option>
        <option value="12am">12:00</option>
    </select>
  <br>
    <select id="Person" name="Person">
        <option value="Person1">Who Would you Like to See?</option>
        <option value="Person2">A Doctor</option>
        <option value="Person3">A Nurse</option>  
    </select>
    <br>
    <input type="submit" data-role="button" id="submit" value="Book" data-icon="action" data-iconpos="right">

I'm not been giving an error message, nor am I getting the success message that i've coded in if the query is successful. Any help would be appreciated

PHP

//This adds the connection file which has the details to connect to the database and what database to connect to
include_once('connection.php');
//Checks if the submit button is not set
if(!isset($_POST['submit']))
{
exit;
}
    //Declared Variables
    $Date = $_GET['Date'];
            $Time = $_GET['Time'];
            $Person = $_GET['Person'];
    $userID= $_SESSION['user'];
    $GPID= $_SESSION['GPID'];


    //Database Connection, this connects to the database using the connection.php
    $conn=ConnectionFactory::connect();

    //Insert Query
    $query="INSERT INTO `Appointments`(`AppID`, `Date`, `Time`, `Booked_With`, `UserID`, `GPID`) VALUES (NULL, :Date,:Time,:Person,:userID,:GPID)";

    $stmt=$conn->prepare($query);

    //Binding values
    //$stmt->bindValue(':post', $Post);
    $stmt->bindValue(':Date', $Date);
    $stmt->bindValue(':Time', $Time);
    $stmt->bindValue(':Person', $Person);
    $stmt->bindValue(':userID', $userID);
    $stmt->bindValue(':GPID', $GPID);

    $affected_rows = $stmt->execute();

    //Message if insert is Successful
    if($affected_rows==1){
        print "<script type=\"text/javascript\">"; 
        print "alert('Post Successful')"; 
        print "</script>";
        exit;
        }

    //Terminates the connection
            $conn=NULL;
?>
     </form>