Date difference from 2 textbox data

Question

Hi,

I have 2 textbox with calender extender binded to them. I am forcing the user to select from date and to date from the calender (format is dd-MM-yy) and i want to display the number of days it comes up to in another textbox which will again be saved as duration in a sql table.. how do i calculate the days? plzz help... Like for ex: i have started the project from 21/6/11 and it will be done till 21/11/11.. i want to calculate the no of days /months and display it in another textbox immediately after the dates for both textbox are selected.. How do i do it plzz help