This guide explains when does a decade actually start in plain language, with the exact figures and how to work them out yourself.
The disagreement is real and old. Because our calendar has no year zero — 1 BC is followed directly by AD 1 — the first decade ran from year 1 to year 10, making every subsequent decade run 1-to-0: 2021–2030, 2031–2040. By that strict count, the “next decade” begins 1 January 2031, and the third millennium began in 2001, not 2000.
Everyday usage disagrees, and reasonably so. “The 2020s” is simply the set of years starting with 202-, from 2020 through 2029 — a naming convention, not an ordinal count, and it needs no year zero. Newspapers, historians of culture, and everyone planning a party use this sense; the great millennium celebration happened on 31 December 1999, whatever astronomers muttered.
Both conventions are internally consistent; they just answer different questions — “which ordinal decade is this?” versus “which named decade is this?” The same tension applies to centuries: the 21st century formally spans 2001–2100, while “the 1900s” ran 1900–1999.
For practical purposes, follow the naming convention and enjoy the round numbers — but if someone insists the decade turns in 2031, they’re not wrong, just answering the other question. The BC/AD converter at itsdately.com handles the no-year-zero arithmetic whenever you need exact spans across the boundary.
Frequently asked questions
Related on itsdately: How to Count 30, 60, or 90 Days From Today · Business Days vs Calendar Days: Why '5 Days' Can Mean a Week and a Half · Q1, Q2, Q3, Q4: Business Quarters Explained
Further reading: U.S. Naval Observatory — calendar FAQs