Problem with calculating difference between 2 textbox - ajax enabled datetime picker

HI

I have 2 textboxes, both of which have been enabled with Ajax datetimepicker calendar tool. The textboxes would display the time and date in this format "dd-mm-yy hh:mm am/pm"      An example of the text would be "06-03-2013 12:00pm" . What I would like to do is, to get the difference between the two dates in the 3rd textbox.

I came across examples that use TimeSpan. I have done this so far

protected void Button1_Click(object sender, EventArgs e)
    {
        DateTime D1 = DateTime.Parse(datepicker.Text);
        DateTime D2 = DateTime.Parse(datepicker1.Text);

        TimeSpan TextBox1 = D2.Subtract(D1);
    }

I think it is not working because of this formatting. How do I correct the code above. I would like to have the code to return the following upon Button1_Click.

Value of 1st Text box = 06-03-2013 12:00pm

Value of 2nd Text box = 06-03-2013 4:30pm

Value of 3rd Text box should be = 4.30