Millennium | Time Period | Britannica

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External Websites

• Humanities LibreTexts - The New Millennium (2000 - 2020)
• National Center for Biotechnology Information - PubMed Central - Simulated millennial-scale climate variability driven by a convection–advection oscillator

millennium time period Ask Anything Homework Help Written and fact-checked by Britannica Editors Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... Britannica Editors Last updated Feb. 21, 2026 •History Britannica AI Ask Anything Table of Contents Table of Contents Ask Anything

millennium, a period of 1,000 years. The Gregorian calendar, put forth in 1582 and subsequently adopted by most countries, did not include a year 0 in the transition from bc (years before Christ) to ad (those since his birth). Thus, the 1st millennium is defined as spanning years 1–1000 and the 2nd the years 1001–2000. Although numerous popular celebrations marked the start of the year 2000, the 21st century and 3rd millennium ad began on January 1, 2001.