function compareError(sender, args) {

          

                var dateStart = $("#<%=txtStartTime.ClientID %>").val().replace(/\-/g, "\/");

                var dateEnd = $("#<%=txtEndTime.ClientID %>").val().replace(/\-/g, "\/");

                if (dateStart.length > 0 && dateEnd.length > 0) {

                    if (dateStart > dateEnd) {

                        args.IsValid = false;

                    }

                    else {

                        args.IsValid = true;

                    }

                }

                else {

                    args.IsValid = true;

                }   

      }          

 

  <asp:CustomValidator ID="CustomValidator1" runat="server" ClientValidationFunction="compareError" ForeColor="red" ErrorMessage="截止日期大于起始日期！"></asp:CustomValidator>