Learn how to calculate the difference between two dates in JavaScript, and in the process, derive some useful applications such as counting down to a particular event, counting up, displaying what's new on your page etc.
There are many ways to add a date on your screen. This page shows how to put a date and day (for instance: wednesday, march 20th, 2002), using an array.
You could use your status bar to display the time and date. You might also use it to display mouse-over text or a welcome message. View the source of the demo page for the source code.