Popup or flat calendar for selecting date/time. Can show week numbers. The look is fully customizable by CSS. It can dynamically change month/year and first day of week (any day can be it). You can also select month/year from drop-down menus. Several state-of-the-art color themes. Can display adjacent months. Smart positioning algorithm. Provides date formatting and parsing functions. Shows help tips. Available in lots of languages. Fully documented.