Can one of you experts please help resolve this problem?

I have this js:

1.   <script type="text/javascript">
                   $(window).load(function () {
                       $("#txtFromDate").datepicker();
                       $('#timeStart').timepicker({ showPeriod: true,
                           onHourShow: OnHourShowCallback,
                           onMinuteShow: OnMinuteShowCallback
                       });
2.                     $("#txtToDate").datepicker();
                       $('#timeEnd').timepicker({ showPeriod: true,
                           onHourShow: OnHourShowCallback,
                           onMinuteShow: OnMinuteShowCallback
                       });
                       function OnHourShowCallback(hour) {
                           if ((hour > 20) || (hour < 6)) {
                               return false; // not valid
                           }
                           return true; // valid
                       }
                       function OnMinuteShowCallback(hour, minute) {
                           if ((hour == 20) && (minute >= 30)) { return false; } // not valid
                           if ((hour == 6) && (minute < 30)) { return false; }   // not valid
                           return true;  // valid
                       }
                       $('#btnSearch').on('click', function () {
                           var sDate = $("#txtFromDate").val();
                           var sTime = $("#timeStart").val();
3.                         var eDate = $("#txtToDate").val();
                           var eTime = $("#timeEnd").val();
4.                         var startDate = new Date(sDate + " " + sTime).getHours();
                           var endDate = new Date(eDate + " " + eTime).getHours();
5.                         //Calulate the time difference
                           var hourDiff = endDate - startDate;
                           //alert(hourDiff);
6.                         //Check if hour difference is less than 4 hours and show the message accordingly
                           if (hourDiff < 4) {
                               var r = false; $($("<div>A mininum of 4 hours is required!</div>")).dialog({ closeOnEscape: false, resizable: false, modal: true, open: function (event, ui) { $(".ui-dialog-titlebar-close").hide(); }, buttons: { Close: function () { r = false; $(this).dialog("close"); } }, close: function () { return r; } });
                               return false;
                           }
7.                         //Add the check condition if the user is above the 4 hours time frame
                           if (hourDiff > 4) {
                               var r = confirm("There may be additional fees for going over the 4 hours!");
                               if (r == true) { // pressed OK
                                   return true;
                               } else { // pressed Cancel
                                   return false;
                               }
                           }
                       });
                   });
     </script>

The script above calculates the difference between start time and end time and determines whether a user is below, within or above 4 hour threshold.

It works quite well.

Let's assume this script along with markup is on a page called sender.aspx.

Once a user has selected start time and end time, the difference is stored in a variable called hourDiff.

We will like to pass the value of hourDiff from sender.aspx to receiver.aspx.

To do so, I have tried the following:

1. document.getElementById("gvCustomers_ctl02_hdReserve").href += ( "&amp;hoursdiff=" + hoursDiff );

which gives me the following error:

Error: Unable to get property 'href' of undefined or null reference

Then I tried:

1. document.getElementById("<%=hdReserve.ClientID %>").href += ("&amp;hoursdiff=" + hourDiff);

and that gives me the following error:

'hdReserve' is not declared. It may be inaccessible due to its protection level.

hdReserve is the control ID of the hyperlink I am using to pass values from sender.aspx to receiver.aspx.

I am only having problem on the javascript side.

However, here is the hyperlink with control Id of hdReserve.

1.               <asp:HyperLink ID="hdReserve" style="color:#AA0000" runat="server" Text="Select"
                   Navigateurl='<%# String.Format("ReserveFacility.aspx?id={0}&groupsize={1}", Eval("siteId")) %>' />

Any ideas to resolve this?

Thank you so much in advance