Decade calculator

Places a date in its decade, quarter and week. The decade question has a pedantic answer and a practical one, and they disagree by a year.

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. In practice nobody uses that: 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. Both are defensible; the digit-based one is what people mean.