For your follow-up question:
When did this century start? 2001 or 2000?
The answer is yes.
It's up for a debate - the 21st century started on 2000 or 2001.
You say:
I would argue to follow the ISO 8601 standard. That way, decades are associated with centuries, so the first decade of this millennium (2000-2009) is entirely in the 21st century, rather than being split over the 20th and 21st.
Here's an example question: Did the 2000s decade run from 2000-2009 or 2001-2010?
Again, the answer is yes.
If you start counting from 1BC, the 0s decade (first AD decade) contains the years 1BC, 1, 2, 3, 4, 5, 6, 7, 8, 9. This makes 10 years. Then the 2000s decade contains the years 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009. Again, 10 years.
However, if you start counting from 1, the 0s decade then contains the years 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. We still have 10 years. The 2000s decade then contains the years 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010.
A century is a period of 100 years - which means it has 10 decades. If you start counting from 1BC, the 1st century will be made of:
0s (1BC-9)
10s (10-19)
20s (20-29)
30s (30-39)
40s (40-49)
50s (50-59)
60s (60-69)
70s (70-79)
80s (80-89)
90s (90-99)
If you start counting from 1:
0s (1-10)
10s (11-20)
20s (21-30)
30s (31-40)
40s (41-50)
50s (51-60)
60s (61-70)
70s (71-80)
80s (81-90)
90s (91-100)
Then the 20th century ran from 1900-1999 if you start counting from 1BC, or from 1901-2000 if you start counting from 1. The 21st century runs from 2001-2100 if you start from 1, or from 1BC, 2000-2099.
The ELU community appears to define decades, centuries, and millenniums as one-indexed (years are counted from 1) rather than zero-indexed. I prefer to count starting from 1 as well, but if you want to start counting from 1BC, then it's your choice - but, take it elsewhere.
