Fait mathématique: 21315 is an unremarkable number.
Culture générale: 21315 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Date historique: May 10th is the day in 1997 that the Maeslantkering, a storm surge barrier in the Netherlands that is one of the world's largest moving structures, is opened by Queen Beatrix.