At that point, you're creating and writing a different flavor of date picker -- and in static HTML land, there is no clean way to update page content dynamicall

At that point, you're creating and writing a different flavor of date picker -- and in static HTML land, there is no clean way to update page content dynamically based on user input, without some kind of JavaScript library interacting with those elements (talking to the UI) and coordinating input data, displaying new derived data/info based on those selections with snappy, accurate feedback. There are hundreds of ways to support this using JavaScript because the community has cleverly come up with many different flavors of view handling to suit different mental models and preferences. What could be better?