CustomValidator的ErrorMessage属性什么时候显示

Question

  日期格式错误时不显示ErrorMessage="请输入正确的日期  ，怎么回事
<asp:TextBox ID="txtStartDate" runat="server"CssClass="smallInput"></asp:TextBox>
<asp:CustomValidator ID="StartDate" EnableClientScript="false" runat="server"
  OnServerValidate="CValidatorStartDate_ServerValidate" ValidationGroup="Contract"
   ForeColor="Red" ErrorMessage="请输入正确的日期">
</asp:CustomValidator>protected void StartDate_ServerValidate(object source, ServerValidateEventArgs args)
        {
            args.IsValid = CommonHelper.DateTimeValidate(txtStartDate.Text.Trim());
        }
 protected void btnSearch_Click(object sender, EventArgs e)
        {
           
            Page.Validate ("Contract");
            if (Page.IsValid ==false)
            {
                return;
            }            setCollectionParametersFromControl();
}