A few wishes:
My Two Favourite for Clean UI:
by Jon Thornton
Take a look at Jon Thornton's latest work, DatePair, which is inspired by Google Calendar's clean UI. From what I've found it's by far the cleanest design and easiest to use.
by Amsul
Amsul's Pickadate.js is super clean and responsive, so it works really well on mobile devices as well, consuming the full screen on small screens.