2001-2100
AD 2015 was in the 21st century
The 21st Century AD.
It is the 21st century
The 21st century AD.
Both. It was the last year of the 21st century BC, and the first year of the 21st century AD.
2004 is in the twenty-first century.
The 21st century started on January 1, 2001 and will continue until December 31, 2100. The 21st century is the first century of the 3rd millennium.
18th century So 2012 is 21st century, etc.
1866 was in the 19th century. 1966 was in the 20th century. 2066 will be in the 21st century.
2001- 3000
The 21st century began in 2001 because the Gregorian calendar, which is the most widely used calendar system today, counts years from the year 1 AD. Since there is no year "0," the first century spanned from 1 AD to 100 AD, the second from 101 AD to 200 AD, and so forth. Consequently, the 20th century ended on December 31, 2000, and the 21st century began on January 1, 2001.
the first century. AD is after death, and anything up to 99 AD is the first century. then comes the second, third, fourth, etc. (technically we live in the 21st century AD).