UpdatePanel里面的DetailsView里面的内容提交后怎样保持原来的值？

Question

代码如下：
<atlas:ScriptManager ID="ScriptManagerMemberList" runat="server" EnablePartialRendering="true">
</atlas:ScriptManager>
                        
    <h3>在这里插入新的记录:</h3>
    
    <atlas:UpdatePanel ID="UpdatePanelDetailsView" runat="server">
        <ContentTemplate>
            <asp:DetailsView ID="DetailsView1" runat="server" AutoGenerateRows="False" DefaultMode="Insert" DataKeyNames="Id" DataSourceID="ObjectDataSource1" >               
                <Fields>
                    <asp:TemplateField>
                        <EditItemTemplate>
                            <asp:Calendar ID="Calendar1" runat="server" SelectedDate='<%# Bind("Operate_Time") %>'>
                            </asp:Calendar>
                        </EditItemTemplate>
                    </asp:TemplateField>
                </Fields>
            </asp:DetailsView>
        </ContentTemplate>
    </atlas:UpdatePanel>我在Page_Load()里面给了Calendar1设定一个初始值：
if (IsPostBack == false)
        {
            Calendar calendar = DetailsViewMaterialList.FindControl("Calendar1") as Calendar;
            calendar.SelectedDate = DateTime.Today;
        }问题：
    为何每次提交后Calendar1的SelectedDate的值都没有了呢？