Jquery Hidden Field

Because jQuery knows nothing about asp:HiddenField. It looks in the HTML structure where you have <input type="hidden" name="ctl00$cph_main$HiddenFieldServerDateTime" id="ctl00_cph_main_HiddenFieldServerDateTime" .... So there's no input with ID= HiddenFieldServerDateTime. There are a few ways to overcome this:

• Use a css selector:

  <asp:HiddenField ID="HiddenFieldServerDateTime" 
                   runat="server" 
                   CssClass="SomeStyle" />
  

  with the following selector: var serverDateTime = $(".SomeStyle").val();

  CssClass is not an available class on the HiddenField class (and it doesn't have an Attributes collection, so you can't add it manually).

• Use ClientID property:

  var serverDateTime = $("#<%= HiddenFieldServerDateTime.ClientID %>").val();
  

• Wrap the hidden field in something you can select:

  <div class="date-time-wrap">
    <asp:HiddenField ID="..." runat="server" />
  </div>
  

   

  var serverDateTime = $('.date-time-wrap input[type=hidden]').val();
  

<input type="hidden" ID="HiddenFieldServerDateTime" runat="server" class="HiddenFieldServerDateTime" />