Hi,
You have to check the following:
1. Check hidden field present.
2. Whether you are writing javascript in separate JS file.
If you are are writing javascript in separate JS file then you cannot get client id directly there.
Just follow the steps:
1. Add the attribute in page load for hidden field.
YourBtn.Attributes.Add("onclick", "return YourFunction('" + HiddenFieldMinutes.ClientID + "')");
2. write the javascript function as follows:
function test(HdnFldID) {
var digiclock = document.getElementById(HdnFldID).value;
}
All the best.
--Amit