The datetime package is one of the best things about Python, and dare I say one of the best general purpose calendar modules ever written. It's just so practical.

>     4_100_000_000 - 2099-12-03 16:53:20

We're going to party like it's 2099! Watch out for that Y2K + 100 bug!

I can already hear people from 2033 celebrating 2 billion.

> leap seconds

Over long periods of time the approximation that a day takes 86400 SI seconds becomes less and less accurate as the rotational period of the Earth changes. I wish calendars would be either purely astronomical in nature or purely SI in nature. Hybrid systems like UTC become more and more messy over time as the amount of adjustment needed increases. We've had ~25 leap seconds in UTC already, and it's a relatively young calendar system.

I also wish we would change the name of the SI measurement "second". An SI second and an astronomical second are two different things, and deserve two different names.