我用ext.net DateField时间控件在第一次加载页面的时候，它自动获取“0001/01/01 00:00:00”这个时间，怎么让它默认 为空， 没有选择的时候在后台也为空， 时间格式：Format="yyyy-MM-dd"是这样的 喜页面显示“2013-02-20” 但是在后台获 取的时候就变成了“2013/2/20 00:00:00”

求解决：
代码：
<ext:Panel ID="Panel4" runat="server" Border="false" Layout="Form" ColumnWidth=".5" Width="300px">
                            <Items>
                                <ext:DateField ID="DateFieldKaiShi" runat="server" Vtype="daterange" FieldLabel=" 开始时间" Format="yyyy-MM-dd">  
                                    <CustomConfig>
                                        <ext:ConfigItem Name="endDateField" Value="DateFieldJieShu" Mode="Value" />
                                    </CustomConfig>
                                    <Listeners>
                                        <KeyUp Fn="onKeyUp" />
                                    </Listeners>
                                </ext:DateField>
                            </Items>
                        </ext:Panel>
                        <ext:Panel ID="Panel5" runat="server" Border="false" Layout="Form" ColumnWidth=".5" Width="300px" >
                            <Items>
                                 <ext:DateField ID="DateFieldJieShu" runat="server" Vtype="daterange" FieldLabel=" 结束时间" Format="yyyy-MM-dd">    
                                    <CustomConfig>
                                        <ext:ConfigItem Name="startDateField" Value="DateFieldKaiShi" Mode="Value" />
                                    </CustomConfig>
                                    <Listeners>
                                        <KeyUp Fn="onKeyUp" />
                                    </Listeners>
                             </ext:DateField>
                            </Items>
                        </ext:Panel>