Decade calculator
Places a date in its decade, and gives the span that decade covers. The decade is the year with its last digit dropped — 1994 sits in the 1990s because 199 is the shared prefix — which is the definition everyone actually uses and not the one the calendar strictly implies.
How to use the decade calculator
Strictly, decades run from year 1 to year 10; the 1990s would be 1991 to 2000, because the calendar has no year zero and counting starts at one. Year 1 BC is followed directly by year 1 AD, so the first decade AD ran 1 to 10 and every decade since has inherited that offset. In practice nobody uses it. Culturally the nineties are 1990 to 1999, defined by the shared leading digits, and that is how everyone refers to them. The strict version resurfaces only in arguments about when a century begins, which is why 1 January 2000 was celebrated as the millennium while the pedants pointed at 2001. This page uses the digit reading — floor the year to a multiple of ten — because that is what someone asking which decade a photograph came from means.
Astronomers took the other route rather than living with the gap: astronomical year numbering does have a year 0, which is 1 BC, so 2 BC is year −1. It exists so that arithmetic across the boundary works without a correction, and it is why a date given by an astronomer can sit a year off the same date given by a historian.
The named decade is a looser thing again. The "sixties" as a cultural period is usually dated from about 1963 to 1974, and the "long nineteenth century" runs 1789 to 1914. Those are arguments about events, not about digits, and no calculator settles them.
What people use it for
- Labelling an archive photograph by decade rather than by year
- Settling whether 2020 belongs to the 2010s or the 2020s
- Naming an era for a themed party or a playlist
- Working out how many years are left in the current decade
- Reconciling an astronomer’s year number with a historian’s BC date
Questions
Strictly 1991, because there is no year zero. Culturally 1990, and that is what people mean and what this page reports.
The 2020s, on the digit reading used here. On the strict reading it is the last year of the 2010s.
The system was devised before zero was in common use in European mathematics. Year 1 BC is followed directly by year 1 AD.
Yes. Astronomical year numbering sets year 0 equal to 1 BC so that arithmetic across the boundary works, which puts it one out from historical dates.
Strictly 1 January 2001. It was celebrated a year earlier because the digits rolled over.
Not as people use it. The cultural sixties is usually dated from around 1963 to 1974, which is a claim about events rather than about digits.