I get NaN value to caluclate the time difference between two datetime.

I use visual studio default website template. I'm developing some application on my own. In that date difference is a part of it.
 I have date pickers to pick the date and i have created drop-down-lists for time. I want a difference between reported date and rectified date. I have written a javascript code for the same onmouseover. But I'm getting NaN value all the time, no errors too. I know NaN is not a legal number, but just want to know what the problem in code.
Please find the piece of code below for the same.
<asp:Content ID="Content1" ContentPlaceHolderID="HeadContent" Runat="Server"> <link rel="stylesheet" href="http://code.jquery.com/ui/1.10.3/themes/smoothness/jquery-ui.css" /> <link rel="stylesheet" href="/resources/demos/style.css" /> <script src="http://code.jquery.com/jquery-1.9.1.js" type="text/javasc ...