Use
OnSelectedIndexChanged event of DropDownList. Try this:
protected void ddlWarrenty_SelectedIndexChanged(object sender, EventArgs e)
{
if(ddlWarrenty.SelectedItem.Text.Trim() == "1 Year")
{
DateTime dtday = Convert.ToDateTime(dptPurchaseDate.Text.Trim());
dptPurchaseDateEx.Text = (Convert.ToInt32(dtday) + Convert.ToInt32(dtday.AddYears(1))).ToString();
}
}
--Amit