Part of my job involves maintaining a Web application which I wrote almost entirely from scratch. I designed the database, wrote pretty much all of the PHP and HTML, and even put my toe in the water of CSS. A colleague now supports part of the application, but v1.0 was more or less entirely my own work.
Anyway, about a month ago I was on a hotel's site and I noticed that the date picker appeared and disappeared in an instant. (It certainly wasn't the first one I've seen like that, but presumably I was in a receptive mood.) It occurred to me that it was running in a DIV rather than a window. I thought this was brilliant and decided to write my own, so that my application could be "pure" (and people wouldn't open 35 tabs trying to specify a date).
The result is called AnyDatePicker. I don't know what this means, except that it does let you pick any date in any year from 1582 (the start of the Gregorian calendar) through 9999. It also has a lot of ways to allow you to control exactly which dates and times people can choose; for example, it would be pretty good for a system where people book 15-minute appointments and you have different opening hours for Fridays, weekends, and holidays.
Please feel free to check it out.