Calculate difference between two Dates

public void date() {
    Datetime startdate;
    Datetime enddate;
    Timespan remaindate;
    startdate = DateTime.Parse(txtstartdate.Text).Date;
    enddate = DateTime.Parse(txtenddate.Text).Date;
    Remaindate = enddate - startdate;
    if (remaindate != null) {
        lblmsg.Text = "you have left with " + Remaindate.TotalDays + "days.";
    } else {
        lblmsg.Text = "correct your code again.";
    }
}
protected void btncal_Click(object sender, EventArgs e) {
    date();
}