Fatto generale: 21003 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21003 is an uninteresting number.
Trivia: 21003 is a boring number.
Data storica: July 2nd is the day in 963 that the imperial army proclaims Nicephorus Phocas Emperor of the Romans on the plains outside Cappadocian Caesarea.