en.wikipedia.org
en.wikipedia.org
timeanddate.com
en.wikipedia.org
scientificamerican.com
Scientific American
https://www.scientificamerican.com › article › when-is-the-beginning-of
simple.wikipedia.org
aa.usno.navy.mil
US Naval Observatory Astronomical Applications Department
https://aa.usno.navy.mil › faq › millennium
hko.gov.hk
Hong Kong Observatory
https://www.hko.gov.hk › en › gts › time › centy-21-e.htm
reference.com
kids.kiddle.co
Kids encyclopedia
https://kids.kiddle.co › 21st_century
almanac.com
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