1000 Day Birthday Calculator [JavaScript]
We don't get enough birthdays, and the ones that we do get are always inconveniently at the same time of year. This little toy tells you when your next 1000 day birthday falls. Any excuse for a party …
Bugs
The layout could be prettier.
Concept
I've been playing about with this idea since 1993. All it is is some simple maths on Julian dates. JavaScript makes this pretty trivial, and easy to present on the web.
Feel free to start some class of religion based on it, but don't forget who told you about it.