Problem in converting string to datetime

Hi,

I am using calender extender for 2 textboxes(txtstartdate,txtenddate) and have set the format as "dd/MM/yyyy". At the txtenddate textchanged event i am calculating no of days to diplay in txtduration.text..I am getting format exception like this:

String not recognised as valid datetime..

Where am i going wrong.. My code is as below

 private int duration(string startdate, string enddate)
    {

        DateTime dt1= DateTime.Parse(startdate);
        DateTime dt2= DateTime.Parse(enddate);
        //dt1 = Convert.ToDateTime(txtstartdate.Text);
        //dt2 = Convert.ToDateTime(txtenddate.Text);
        TimeSpan ts = dt2.Subtract(dt1);
        int days = ts.Days;
        Session["days"] = days.ToString();
        return days;
    }
    protected void txtenddate_TextChanged(object sender, EventArgs e)
    {
       int projectdays = duration(txtstartdate.Text, txtenddate.Text);

        txtduration.Text = Convert.ToString(projectdays);
    }