Datepicker remains in a fixed place on page

HTML

<link rel="stylesheet" href="http://netdna.bootstrapcdn.com/twitter-bootstrap/2.3.2/css/bootstrap-combined.min.css">
<script src="http://netdna.bootstrapcdn.com/twitter-bootstrap/2.3.2/js/bootstrap.min.js"></script>
<link rel="stylesheet" href="http://eternicode.github.io/bootstrap-datepicker/bootstrap-datepicker/css/datepicker.css">
<script src="http://eternicode.github.io/bootstrap-datepicker/bootstrap-datepicker/js/bootstrap-datepicker.js"></script>
<button class="btn btn-primary" data-toggle="modal" data-target="#myModal">Launch Modal</button>
<div id="myModal" class="modal hide fade" style="height: 400px; overflow: scroll">
    <div style="height:300px"></div>
    <div>Choose Date:
        <input class="calendar" />
    </div>
    <div style="height:300px"></div>
</div>

JavaScript

var datePicker = $(".calendar").datepicker({});