Why is day of the month 1-indexed but the month is 0-indexed in C?

Okay, I put WAY too much time into researching this, and I almost know what's happening. ALMOST.

First of all, JavaScript does this for legacy reasons. In C's <time.h> we have `struct tm`, which does the same thing.

But why?

We see the rabbit hole, time to jump in. https://t.co/fZO3HXfZTN

— Hillel is on vacation (@hillelogram) November 19, 2020

from Hacker News https://twitter.com/hillelogram/status/1329228419628998665