How use hidden field value in updatepanel

Any asynchronous round trips to the server will cause only those controls within the UpdatePanelitself to update on the UI, so even though the code-behind runs and updates the hidden fields, on the front end they stay the same because they sit outside the panel.

Try moving the hidden fields within the <ContentTemplate> tag

<asp:UpdatePanel ID="upTripsGrid" runat="server" UpdateMode="Always">
  <ContentTemplate>
         <input type="hidden" runat="server" id="hidCurrentDate" value="" />
         <input type="hidden" runat="server" id="hidTripIds" value="" />
  </ContentTemplate>
</asp:UpdatePanel>