C# - Selection Change Event Does Not Fired (trigger)

User selects a date from the date-picker and the date is put into a text-box.

But when i choose a date some date should check another text box date and give me the difference. The selection-change event does not fire (as expected).

protected void dtpleavestartdate_SelectionChanged(object sender, EventArgs e)
{
string a = "NO CHANGE";
a = a + dtpenddate.SelectedDate.Subtract(dtpleavestartdate.SelectedDate).ToString();
Response.Write("<script>alert('" + a +"')</script>");
}