Astronomical year numbering
Cassini gave the following reasons for using a year 0:
Fred Espanak of NASA lists 50 phases of the moon within year 0, showing that it is a full year, not an instant in time. Jean Meeus gives the following explanation:
Although he used the usual French terms "avant J.-C." (before Jesus Christ) and "après J.-C." (after Jesus Christ) to label years elsewhere in his book, the Byzantine historian Venance Grumel used negative years (identified by a minus sign, −) to label BC years and unsigned positive years to label AD years in a table. He did so possibly to save space and put no year 0 between them.
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.