Need to calculate the no of days between two dates?

Need to calculate the no of days between two dates?

I had created a jquery calender by using
[url]
http://jqueryui.com/demos/datepicker/
[/url]
I have some thing like this::

Check in Date :: 21/04/2009
Check out Date :: 25/04/2009

I need to calculate the no of days between two dates?
The Result should be something like this :: 4

Could you please help me out of this?