程序代码：

        //-----显示和选择出生日期，登记日期，有效期-----
        private void cmb3_DropDown(object sender, System.EventArgs e)
        {
            selectDate(cmb3);

        }
        private void cmb4_DropDown(object sender, System.EventArgs e)
        {
            selectDate(cmb4);
        }

        private void cmb5_DropDown(object sender, System.EventArgs e)
        {
            selectDate(cmb5);
        }

        //----------关闭日期控件的显示----------
        private void cmb3_Leave(object sender, System.EventArgs e)
        {
            calen1.Visible = false;
        }
        private void cmb4_Leave(object sender, System.EventArgs e)
        {
            calen1.Visible = false;
        }
        private void cmb5_Leave(object sender, System.EventArgs e)
        {
            calen1.Visible = false;
        }

        //-----------单击下拉列表框下拉箭头时显示日期以供选择---------------
        private void selectDate(ComboBox cb)
        {
            calen1.Left = cb.Left;//设置日期控件的位置
            calen1.Top = cb.Top - calen1.Height - 10;
            calen1.SelectionStart = Convert.ToDateTime(cb.Text);//日历显示的时间为数据时间
            calen1.SelectionEnd = Convert.ToDateTime(cb.Text);

            calen1.Visible = true;//显示日期
            calen1.Show();
        }

        //-----------从日历控件中选择日期信息------------
        private void calen1_DateSelected(object sender, System.Windows.Forms.DateRangeEventArgs e)
        {
            //判断是由哪个下拉列表框调出日历，以便回馈日期信息
            if (calen1.Left == cmb3.Left)
                cmb3.Text = calen1.SelectionEnd.ToString();
            if (calen1.Left == cmb4.Left)
                cmb4.Text = calen1.SelectionEnd.ToString();
            if (calen1.Left == cmb5.Left)
                cmb5.Text = calen1.SelectionEnd.ToString();
        }