alt Hacker News> Centuries in the Gregorian calendar start from 00 with their last two digits - 1800, 1900, and 2000.
Umm, wait, what? No, Gregorian centuries most definitely start at year xx01! The first century was from year 1 to 100, the second from 101 to 200, and so on. There was no year zero in the Gregorian calendar.
This is one of those things that cause confusion: There are two sets of centuries, each mostly overlapping but not quite identical.
There’s the numbered centuries: e.g., the twentieth century, which ran from 1901–2000.
Then there’s centuries named by the first two digits: e.g., the 1900s which ran from 1900–1999.
I would note that decades are always indicated by the first three digits and (almost¹) never as numbered decades so when people talk about the ’20s they’re referring to 1920–1929 or 2020–2029 and not, e.g., 2021–2030.²
⸻
1. I’m sure you can find somewhere someone talking about the first decade of the twenty-first century, although that’s likely an attempt to get around the awkwardness of not having convenient names for decades like 2000–2009 or 2010–2019.
2. Every ten years you’ll find tedious people who will insist that the decade doesn’t end in ###9. Don’t argue with them. Just walk away.