https://en.wikipedia.org/wiki/Decade#UsageAny period of ten years is a decade, and there is no 'official' legal nor administrative start or end point,[1][2] so it can be any arbitrary span of ten years. For example, (1) 'the first decade' of a person's life begins on their birthday; (2) the statement that "during his last decade, Mozart explored chromatic harmony to a degree rare at the time" merely refers to the last ten years of Mozart's life without regard to which calendar years are encompassed.
There are two methods of counting decades in recognition. One, called ordinal, counts decades starting with the first year 1 CE (For example, the years 1981–1990 is referred to as the 199th decade), while the other, called cardinal, groups years based on having the same digits (For example, the years 1980–1989 is referred to as the 1980s, or commonly known as the eighties).
A YouGov poll was conducted on December 2, 2019, asking 13,582 Americans whether the following decade would begin on New Year's Day 2020 or New Year's Day 2021. Results show that 64% of Americans answered the next decade will begin on January 1, 2020, and will end on December 31, 2029. 19% of the Americans surveyed replied they are unsure, while 17% answered the next decade will begin on January 1, 2021, and will end on December 31, 2030.[3]
The frequently used method to refer to decades is the cardinal method, which groups years based on their shared tens digit, such as the nineteen-sixties (1960s) referring to the period from 1960 to 1969.[4][5] Sometimes, only the tens part is mentioned (60s or sixties), although this may leave it uncertain which century is meant. In this method, a new decade begins when the third digit of a numerical year changes (For example, the 1970s began when 1969 ended and 1970 took over) while ending on the last day of any year ending in nine. The fact of there being no year zero does not apply, as this method counts decades cardinally rather than ordinally.
The rarer ordinal decade counts years beginning with the year AD 1, as the Gregorian calendar counts ordinally rather than cardinally, and hence there was no year zero. For example, the term 196th decade spans the years from 1951 to 1960. The last year of an ordinal decade ends in zero while matching with the corresponding digits used in the title (For example, the 201st decade spans from 2001 to 2010).
Particularly in the 20th century, a nominal decade is sometimes used to refer not just to a set of ten years but rather to a period of about ten years – for example, the phrase the sixties often refers to events that took place between around 1964 and 1972,[6] and to memories of the counterculture, flower power, protests of 1968 and other things happening at the time. Often, such a nominal decade will come to be known by a title, such as the "Swinging Sixties" (1960s), the "Warring Forties" (1940s) and the "Roaring Twenties" (1920s). This practice is occasionally also applied to decades of earlier centuries, for example, references to the 1890s as the "Gay Nineties" or "Naughty Nineties".