C# 日期选择器回发返回原始值

我有这样的日历：

控制器：

  protected void Page_Load(object sender, EventArgs e)
        {

 fecha_fin.Text = DateTime.Today.ToString("yyyy/MM/dd", CultureInfo.InvariantCulture);
                    Thread.CurrentThread.CurrentCulture = new CultureInfo("es-MX");

 }


  protected void DateChange(object sender, EventArgs e)
        {
            fecha_fin.Text = Calendar1.SelectedDate.ToString("yyyy/MM/dd");
        }

  <td>
                <asp:TextBox ID="fecha_fin" ClientIDMode="Static" runat="server"></asp:TextBox>
                <asp:Calendar ID="Calendar1" runat="server" OnSelectionChanged="DateChange" DayNameFormat="FirstLetter" Font-Names="Tahoma" Font-Size="11px" NextMonthText="." PrevMonthText="." SelectMonthText="»" SelectWeekText="›" CssClass="myCalendar" CellPadding="0">
                    <OtherMonthDayStyle ForeColor="#b0b0b0" />
                    <DayStyle CssClass="myCalendarDay" ForeColor="#2d3338" />
                    <DayHeaderStyle CssClass="myCalendarDayHeader" ForeColor="#2d3338" />
                    <SelectedDayStyle Font-Bold="True" Font-Size="12px" CssClass="myCalendarSelector" />
                    <TodayDayStyle CssClass="myCalendarToday" />
                    <SelectorStyle CssClass="myCalendarSelector" />
                    <NextPrevStyle CssClass="myCalendarNextPrev" />
                    <TitleStyle CssClass="myCalendarTitle" />
                </asp:Calendar>

            </td>

查看：

  protected void Page_Load(object sender, EventArgs e)
        {

 fecha_fin.Text = DateTime.Today.ToString("yyyy/MM/dd", CultureInfo.InvariantCulture);
                    Thread.CurrentThread.CurrentCulture = new CultureInfo("es-MX");

 }


  protected void DateChange(object sender, EventArgs e)
        {
            fecha_fin.Text = Calendar1.SelectedDate.ToString("yyyy/MM/dd");
        }

  <td>
                <asp:TextBox ID="fecha_fin" ClientIDMode="Static" runat="server"></asp:TextBox>
                <asp:Calendar ID="Calendar1" runat="server" OnSelectionChanged="DateChange" DayNameFormat="FirstLetter" Font-Names="Tahoma" Font-Size="11px" NextMonthText="." PrevMonthText="." SelectMonthText="»" SelectWeekText="›" CssClass="myCalendar" CellPadding="0">
                    <OtherMonthDayStyle ForeColor="#b0b0b0" />
                    <DayStyle CssClass="myCalendarDay" ForeColor="#2d3338" />
                    <DayHeaderStyle CssClass="myCalendarDayHeader" ForeColor="#2d3338" />
                    <SelectedDayStyle Font-Bold="True" Font-Size="12px" CssClass="myCalendarSelector" />
                    <TodayDayStyle CssClass="myCalendarToday" />
                    <SelectorStyle CssClass="myCalendarSelector" />
                    <NextPrevStyle CssClass="myCalendarNextPrev" />
                    <TitleStyle CssClass="myCalendarTitle" />
                </asp:Calendar>

            </td>

这里我选择了10月31日，但文本返回12月12日。为什么页面重新加载回发为原始页面（页面加载时）？问候

在页面加载时检查其是否不是页面加载。。这不会更改回发上的值

如果（！IsPostBack）