TypeError: date undefined in bootstrap-datepicker

I am using bootstrap datepicker from http://www.eyecon.ro/bootstrap-datepicker/

But firebug shows the undefined date in the bootstrap-datepicker.js file on the next line.

var parts = date.split(format.separator)

Javascript file and css work fine

<script type="text/javascript" src="../bootstrap/js/bootstrap-datepicker.js"></script>

<link rel="stylesheet" href="../bootstrap/css/datepicker.css" type="text/css"/>

And the date selection was initialized using

$('.datepicker').datepicker({ format: "yyyy-mm-dd" });