Designed to be a part of the Standard Computer Interaction Library I am developing as a personal project.
If you use this please know what you are doing or purchase some of my time so I can help you understand how to fit this into your project
This is how JS is meant to be done!
Another in the Pure CSS series of widgets a pure-css calendar (would require being generated by backend language to avoid JS)
using form controls checkbox & radio to enable accordion toggles in pure CSS