Technically, decades and centuries start in a January with one or two zeros at the end, respectively. So the 1700s and the 18th century are exactly the same int

Technically, decades and centuries start in a January with one or two zeros at the end, respectively. So the 1700s and the 18th century are exactly the same interval of time. ISO 8601-2: > Decade: A string consisting of three digits represents a decade, for example “the 1960s”. It is the ten-year time interval of those years where the three specified digits are the first three digits of the year. > Century: Two digits may be used to indicate the century which is the hundred year time interval consisting of years beginning with those two digits.