Decade indication

Does the next decade start on 2020-01-01 or 2021-01-01? Postgres says
it start on the former date:

SELECT EXTRACT(DECADE FROM '2019-01-01'::date);
date_part
-----------
201

SELECT EXTRACT(DECADE FROM '2020-01-01'::date);
date_part
-----------
202

but the _century_ starts on 2001-01-01, not 2000-01-01:

SELECT EXTRACT(CENTURY FROM '2000-01-01'::date);
date_part
-----------
20

SELECT EXTRACT(CENTURY FROM '2001-01-01'::date);
date_part
-----------
21

That seems inconsistent to me. /pgtop/src/backend/utils/adt/timestamp.c
has this C comment:

* what is a decade wrt dates? let us assume that decade 199
* is 1990 thru 1999... decade 0 starts on year 1 BC, and -1
* is 11 BC thru 2 BC...

FYI, these two URLs suggest the inconsistency is OK:

https://www.timeanddate.com/calendar/decade.html
https://en.wikipedia.org/wiki/Decade

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ As you are, so once was I. As I am, so you will be. +
+ Ancient Roman grave inscription +