A decade is defined as 10 years and the word itself derives from the Greek word dekas, meaning a group of 10.
A decagon is a shape with exactly 10 sides and a decagram is exactly 10 grams.
But because the Gregorian calendar started at year 1 AD, the first decade officially lasted from year 1 AD to 11 AD and not 10 AD.
There is, however, some additional confusion when people discuss specific decades such as the 1920s or the 1960s.
And a YouGov poll published on December 2, 2019, has found 64 percent of those polled consider 2020 to be the start of the next decade.
source: express.co.uk