日期格式错误时不显示ErrorMessage="请输入正确的日期  ，怎么回事，是不是当isvaild为false时就应该显示错误信息
<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();
}