Did we “entered the 21st century” in 2000?
Yes. Just like when a baby is born it has begun the first year of its life. Well it's a really dumb interpretation if it has to start in 2001 just because they didn't have a year Zero when they came up with the idea of a calendar.
The new millennium started in 2000, 21st century started 2001.
Spirit of All
2001 because it has a 1 in it. The 20th century would be the year 2000.