What's My Birthday?

The app referenced is a nice encapsulation of the formula in a form that is immediately usable without significant understanding.

If you find this interesting and want to know how these things are calculated, you can find the full calculations in a blog post I wrote a few years ago:

https://www.solipsys.co.uk/new/TheBirthdayParadox.html?si21h...

There are some previous discussions:

https://news.ycombinator.com/item?id=19296265

https://news.ycombinator.com/item?id=4753014