AutoPostBack = true会影响日期文本框的更改

Question

我的网页中有以下组件：

 <div class="formrows">
            <span class="fieldlable">Report Period:</span>
            <span>
                <asp:TextBox ID="TextBoxDateStart" ClientIDMode="Static" runat="server" Text="<%# BindItem.DateStart %>" CssClass="textbox" /></span>
            <span>To:</span>
            <span>
                <asp:TextBox ID="TextBoxDateEnd" ClientIDMode="Static" runat="server" Text="<%# BindItem.DateEnd %>" CssClass="textbox" /></span>
            <asp:RequiredFieldValidator ID="RequiredFieldValidatorStartDate"   Display="None" runat="server" ErrorMessage="Please enter Start Date"  ValidationGroup="validate" ControlToValidate="TextBoxDateStart"></asp:RequiredFieldValidator>    
            <asp:RequiredFieldValidator ID="RequiredFieldValidatorEndDate"   Display="None" runat="server" ErrorMessage="Please enter End Date" Text="" ValidationGroup="validate" ControlToValidate="TextBoxDateEnd"></asp:RequiredFieldValidator>    
            <asp:RegularExpressionValidator ID="RegularExpressionValidatorStartDate" runat="server"   Display="None"  ErrorMessage="Please Start Date in the correct format (mm/dd/yyyy)" ValidationGroup="validate" ControlToValidate="TextBoxDateStart" ValidationExpression="(19|20)\d\d[- /.](0[1-9]|1[012])[- /.](0[1-9]|[12][0-9]|3[01])"></asp:RegularExpressionValidator>
            <asp:RegularExpressionValidator ID="RegularExpressionValidatorEndDate" runat="server"   Display="None"  ErrorMessage="Please End Date in the correct format (mm/dd/yyyy)" ValidationGroup="validate" ControlToValidate="TextBoxDateEnd" ValidationExpression="(19|20)\d\d[- /.](0[1-9]|1[012])[- /.](0[1-9]|[12][0-9]|3[01])"></asp:RegularExpressionValidator>
            <asp:CustomValidator runat="server"  EnableClientScript="false" ID="valDateRange"  Display="None" ValidationGroup="validate" ControlToValidate="TextBoxDateStart" onservervalidate="valDateRange_ServerValidate" ErrorMessage="Start Date and End Date must within one month" />
            <asp:CustomValidator runat="server" EnableClientScript="false"  ID="CustomValidatorEndDate"  Display="None" ValidationGroup="validate" ControlToValidate="TextBoxDateEnd" onservervalidate="valEndDateRange_ServerValidate" ErrorMessage="Start Date must earlier than End Date" />

        </div>
            
         <div class="formrows">
            <span class="fieldlable">Country:</span>
            <span>
                <asp:DropDownList ID="ddlCountry" runat="server" DataTextField="Text" DataValueField="Value" AutoPostBack="true" SelectedIndex="<%# BindItem.CountryID %>" SelectMethod="LoadCountry"  OnSelectedIndexChanged="ddlCountry_SelectedIndexChanged"></asp:DropDownList>
            </span>
            <asp:RequiredFieldValidator ID="RequiredFieldValidatorCountry" runat="server" ErrorMessage="Country" ControlToValidate="ddlCountry"></asp:RequiredFieldValidator>    

        </div>

例如，

当我写日期文本框选择日期2017/02/01然后我选择其他国家时，日期将更改为今天日期。

对于后端源代码，我已添加：

if (!IsPostBack == true)
{
    ...

但它根本没有效果。

我可以知道如何解决它吗？