Fatto generale: 21916 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21916 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Trivia: 21916 is an uninteresting number.
Data storica: January 1st is the day in 1801 that the legislative union of Kingdom of Great Britain and Kingdom of Ireland is completed to form the United Kingdom of Great Britain and Ireland.