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.