Popup or flat calendar for selecting dates. Can show week numbers. The look is customizable by CSS so you don't need to modify the JS code. It can dynamically change month/year and first day of week (toggle between Mon/ Sun). You can also select month/year from a drop-down menu. The ZIP file contains several color themes. Provides date formatting and parsing functions. It has a nice status bar that shows help tips. Very easy to internationalize (and already translated in 24 languages). Fully documented. Supports time selection.