A topic I always like to play with in software are date/time/timezone things. You might ask why, as it is a constant pain ...

The reason is that it is a fascinating combination of the three pillars:

* Nature is the cause of it (earth turns in a specific speed and travels in a specific speed around the sun)
* Human culture brought us time measurement, timezones, daylight saving time, ...
* computers are pure maths machines requiring clear logic

I'm always fascinated how those interweave.