Hecho general: 21817 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Hecho matemático: 21817 is an unremarkable number.
Trivia: 21817 is an uninteresting number.
Fecha histórica: September 24th is the day in 1852 that the first airship powered by (a steam) engine, created by Henri Giffard, travels 17 miles (27 km) from Paris to Trappes.