Yellow5 wrote a unobtrusive, lightweight javascript datepicker for us. It is so small that you can use it on your pda, but the functionality is just about right. There are also a few advantages to using DateChooser over other javascript datepickers according to Yellow5:

• The HTML for the calendar is standards-compliant.
• The calendar appears in the same window as the input field(s). It doesn’t annoyingly pop up in a new window.
• It has no hard-coded styles; apply your own .css file to customize DateChooser’s appearance.
• Return values are flexible, using patterns borrowed from PHP’s date() function.
• It doesn’t pollute the global namespace.
• It uses unobtrusive, warning/error-free javascript.

Requirements: Internet Explorer 5+, Firefox 1.0+, Opera 8+, Safari
Demo: http://yellow5.us/projects/datechooser
License: CC Attribution 2.0 License