Year zero
Historians have never included a year zero. This means that between, for example, and , there are 999 years: 500 years BC, and 499 years AD preceding 500. In common usage "anno Domini" 1 is preceded by the year 1 BC, without an intervening year zero. Neither the choice of calendar system (whether Julian or Gregorian) nor the era ("Anno Domini" or Common Era) determines whether a year zero will be used. If writers do not use the convention of their group (historians or astronomers), they must explicitly state whether they include a year 0 in their count of years, otherwise their historical dates will be misunderstood.
2000s (decade)
In the English-speaking world, a name for the decade was never universally accepted in the same manner as for decades such as the '80s, the '90s, etc. Orthographically, the decade can be written as the "2000s" or the 00s". Common suggestions for referring to this decade: "2000s", "Two-thousands", "Twenty Hundreds", "Twenty-ohs", "00s" (pronounced "Ohs", "Oh Ohs", "Double Ohs" or "Ooze", "Zeros", "Double Zeros"), "the Aughts", "the Noughties", "the Aughties", "the Oughties". Other suggestions from 45 countries suggest the "double nothings", "zilches", "oh-zone", "oh-something". When the "20–" is dropped, the individual years within the decade are usually referred to as starting with an "oh", such as "oh-seven" to refer to the year 2007. During the decade of the 2000s, it was more common to hear years referred to starting with "two-thousand (and)" rather than "twenty-oh". Starting around the middle of the 2010s, it is becoming more common to refer to the individual years of the decade as "twenty-oh-seven" or "twenty-oh-eight" than it had been during the 2000s, although the "two thousand (and)" pattern is still dominant.