5.89 years, decades, and centuries

Years are referred to in English by numbers.

To refer to periods longer than a year, decades (ten years) and centuries (a hundred years) are used. Decades start with a year ending in zero and finish with a year ending in nine: 'the 1960s' (1960 to 1969), 'the 1820's' (1820 to 1829). If the century is already known, it can be omitted: 'the 20s', 'the twenties', 'the Twenties'.

To be more specific, for example in historical dates, 'AD' is added before or after the numbers for years or centuries after Christ is believed to have been born: '1650 AD', 'AD 1650', 'AD 1650-53', '1650-53 AD'. 'BC' is added after the numbers for years or centuries before Christ is believed to have been born: '1500 BC', '12-1500 BC'.

Centuries start with a year ending in two zeroes and finish with a year ending in two nines. Ordinals are used to refer to them. The 'first century' was from '0 AD' to '99 AD', the 'second century' was '100-199 AD', and so on, so the period '1800-1899 AD' was the 'nineteenth century' and we are currently in the 'twentieth century' (1900-1999 AD). Centuries can also be written using numbers: 'the 20th century'.