As long as you can uniquely identify an element - you can access it (the fact that it is a hidden field is irrelevant, as it is only for the visual part is hidden, but still there in the DOM)...
ASP.NET HiddenField renders to a HTML element of input with type set to hidden...The only problem is that the id can be extended by the ASP.NET rendering engine...Learn about it
here[
^]...
Now the way to get the generated id from client side code is using inline server tags...
Plain JS
var el = document.getElementById('<%= hdnUsedTime.ClientID %>');
JQuery
var el = $('#<%= hdnUsedTime.ClientID %>');
The <%= hdnUsedTime.ClientID %> part will be translated to the real, client-side id of the element when page got rendered...