A decade is a period of time defined as 10 years. It can refer to any period of ten years, be it years in a person's life, or years on a calendar. The following are all examples of a decade:

Counting decades

There are two different methods used when defining a decade using the Gregorian calendar, the standard calendar used around most of the world. In one method, the years are counted from 0 to 9, and in the other, the years are counted from 1 to 0.

0 to 9

Although both the 0 to 9 and 1 to 0 methods are valid, the 0 to 9 method is more widely used. In the 0 to 9 method, we count the decade starting from years ending in 0 up till a year ending in 9. For example, using this method, the year 2000 marked the beginning of a new decade, which ended with the year 2009. Technically the Gregorian calendar does not have a year 0, but for the purposes of this method, it is assumed.

According to a 2019 poll, 64% of US adults thought that the new decade would begin Jan 1, 2020, 17% thought it would begin Jan 1, 2021, and the remaining 19% did not know. It is possible that the 0 to 9 method is more prevalent because culturally, we have used terms such as "the 80s" to group periods of 10 years. It makes more sense that the 1980s would include the years 1980-1989, rather than 1981-1990 even though both are correct; it simply depends on how we choose to define the decade.

1 to 0

This method groups years with the first year of the decade ending in a 1, and the last year in the decade ending in a 0. Based on this method, the first decade in the Gregorian calendar was the decade that began on the year 1, and ended with the year 10. The next decade began on the year 11 and ended with the year 20, and so on. In this method, the year 2020 is not the beginning of the new decade, and the new decade will start on Jan 1, 2021.