Store date picker date and save in database

Im trying to get the date from a date picker i have on my page and then have that date posted to a php function which will store the selected date in my database.

This is the code i am working with now but i am having no luck storing the date in a variable and then passing it into the

//php script where i want the date to become available

if(empty($_POST)===false){

    $data = array(
        'employee' => $user_data ['first_name'],
        'store' => $user_data ['store'],
        'noofdays' => $_POST['quantity']
        );

    //book_holiday($data);
    header('Location: index.php');
    exit();
}

//script

<script>

var date;

date= $('#datepicker').datepicker({
              format: 'mm-dd-yyyy'
            });

</script>

Is there any way i can do this as the way i am trying now will not save the date anywhere.

EDIT:

<form action = "" method = "post">

<div id="datepicker" style = "position:relative; TOP:15px;"></div><br><br>

<input type="submit"><br><br></form>