unable to get value of textbox?

So I have a usercontrol which is a date extender type of control.

it contains 2 textboxes for start and end date and for each of these textbox's, it contains the CalendarExtender control.

Now, I have had to make the textbox control public so I can access it via javascript and check the value.

For some reason, it does not detect any value in the StartDate but it does for EndDate even though both textboxes have the value entered.

Any ideas why such a behavior?

Calendar user control:

<div id="divStartDate" style="float:left" runat="server"><asp:Literal runat="server" ID="litStartDateHeading" /><asp:TextBox runat="server" ID="txtStartDate" /><asp:Image runat="server" ID="imgCalendar" ImageUrl="~/Images/Calendar_scheduleHS.png" /><ajaxToolkit:CalendarExtender runat="server" ID="ajaxCalExtStart" Format="yyyy-MM-dd" PopupButtonID="imgCalendar" TargetControlID="txtStartDate" /><ajaxToolkit:TextBoxWatermarkExtender runat="server" ID="ajaxStartWatermark" WatermarkCssClass="watermarked" TargetControlID="txtStartDate" /></div><div id="divEndDate" runat="server" visible="false" style="float:left">&nbsp;<asp:Literal runat="server" ID="litTextBetweenCalendarHeadings" /><asp:Literal runat="server" ID="litEndDateHeading" /><asp:TextBox runat="server" ID="txtEndDate" /><asp:Image runat="server" ID="imgEndDateCalendar" ImageUrl="~/Images/Calendar_scheduleHS.png" /><ajaxToolkit:CalendarExtender runat="server" ID="ajaxCalExtEnd" Format="yyyy-MM-dd" PopupButtonID="imgEndDateCalendar" TargetControlID="txtEndDate" /><ajaxToolkit:TextBoxWatermarkExtender runat="server" ID="ajaxEndWatermark" WatermarkCssClass="watermarked" TargetControlID="txtEndDate" /></div>

Calling from Javascript to get the value:

var calStartDate = document.getElementById("<%= ucCalibrationDueDates.txtStartDate.ClientID %>").value;
var calEndDate = document.getElementById("<%= ucCalibrationDueDates.txtEndDate.ClientID %>").value;

calStartDate returns an empty string meanwhile calEndDate gives me the date the user selected (or typed in).