Nobody is really saying not to build these things. They’re saying the problem is exceedingly annoying to solve—and often not in a technically interesting way but in a way that is just massively tedious—and a better alternative almost certainly already exists.

If you want to build it to scratch an itch, go ahead. If you want to build it for fun, go ahead. If you want to build it because an existing solution gets something wrong and you can do better, go ahead (but know that it is a way bigger undertaking than you might assume at first glance).

The real advice is “don’t casually build your own X”, but that’s less punchy.

An exemplary one is "don't build your own timezone database"

It's not interesting, it's not fun, it's just a process of getting complaints it's wrong in edge cases and then fixing them, over and over until no one can find another broken edge case.

You can start by writing down England is +0, Germany is +2, etc... someone's going to mention DST and you'll put in a field for switching on the Nth Sunday of month X... later you'll run into a country that uses a different rule and you'll add a bunch of spaghetti code or write a Turing-complete DSL, etc... One day someone tells you about a village where they count 17 hour days on seashells and then you'll give up.

And if your DB doesn't produce identical results to the Olson DB in all cases then you created an incompatibility anyway. Might as well just use the Olson DB.

>If you want to build it because an existing solution gets something wrong and you can do better, go ahead

But please at least file the bug first.