Fatto generale: 21176 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21176 is an uninteresting number.
Trivia: 21176 is an unremarkable number.
Data storica: December 22nd is the day in 2001 that Richard Reid attempts to destroy a passenger airliner by igniting explosives hidden in his shoes aboard American Airlines Flight 63.