With 2005 approaching, I started thinking about the ways we mark the passage of time. (E.g., trips around the Sun, the Earth rotating about its axis, etc.)
I stumbled across a site (click here also click here for the home page) that has a "calculator of useless facts." One inputs a date and time, and it calculates what a subsequent date and time is in so many seconds, minutes, hours or days. Areas of the website enable one to mark time in any number of seconds, minutes, hours and days.
For example, it calculates time intervals and presents one with a list of for example 1 billion seconds, 25 million minutes, 1/2 million hours, 25,000 days, etc.
Mathematically inclined children of all ages enjoy this, and it also informs one of the next "celebratory" event. For example, one could use it to plan a surprise party for someone when he or she turns 20,000 days old, or one billion seconds old!