https://github.com/listenrightmeow/daterange
Under 5k/100 lines uncompressed. You can easily modify above to work in a prototype fashion without a library. Unfortunately I was lazy when I wrote it and used ender/jquery for selector support.
* The example looks unappealing. Not styled, not bound to a text field.
* It's not clear to my why clicking twice in a row produces a range — should this not be accomplished through mouse dragging or similar?
* If I select an end date that precedes the start date, it shows an alert box, which apparently is hardwired into the library. Not only does this make it impossible to translate the widget into multiple languages, but it's really not the task of the library to handle the error like this (an "onInvalidRange" or similar should be provided for the app to plug in).
* Month name is hardwired, no translation possible.
* No week numbers.
* No highlighting of today, holidays, or indeed of the selected range.
* No way to plug in custom dates that should be highlighted (for example, "booked" dates).
In other words, this is incomplete. I can't speak for most people, but it's useless to me. Sure, I could take your code and extend it to fit my app, but I could just as easily write it from scratch -- which defeats the point of reusability.
Sorry if this comes across a bit harsh, but you did open yourself to criticism:
> I have always wondered why javascript date pickers were always over complicated for the job.
Frankly, I suspect you have underestimated the job. :-)