When do you want the date updated? When the doctype is created or everytime something changed on the doctype?
For client-side you can refer to this link for the datetime utils that you can use:
you need somethings like this:
var current_datetime = frappe.datetime.now_datetime();
var added_datetime = frappe.datetime.add_days(current_date,2)
for adding hours you’ll probably have to read the momentjs documentations