A few wishes:
It's an old post, but for future reference, I wanted to put this here:
Calentim Date Time Picker Plugin
I've found Trent Richardson's date/time picker to be good. It has a heap of options and seems to work well.
Available here: http://trentrichardson.com/examples/timepicker/
It's based on jQuery UI
The Any+Time(TM) widget combines a date/time picker for a single input field with a wide variety of formats. It now uses jQuery and supports jQuery UI Themes and CSS customization for complete visual integration into any website. And if you encounter any bugs, just report them and expect a resolution in a matter of hours!
Datepicker:
jQuery UI Datepicker
Timepicker:
timepickr plugin
Bonus points, both use the jQuery UI Themeroller.
Check this http://www.queness.com/post/204/25-jquery-plugins-that-enhance-and-beautify-html-form-elements#8
I also stumbled upon the following:
http://code.google.com/p/dyndatetime/
http://www.kelvinluck.com/assets/jquery/datePicker/
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.