assuming you parse the string into JavaScript date object, you can do (date2 - date1)/1000
to parse mysql format timestamp, just feed the string into new Date():
var d2 = new Date('2038-01-19 03:14:07');
var d1 = new Date('2038-01-19 03:10:07');
var seconds = (d2- d1)/1000;
fix of edit 2 in the question:
<script type="text/javascript">
$(function(){
var d2 = new Date();
var d1 = new Date("2014-02-02 23:54:04");
$("a#timedif").html("Diff. Seconds : "+((d2-d1)/1000).toString());
});