15821009

This number is a prime.

The date 1582/10/09 (YYYY/MM/DD) is invalid (check the iPhone's calendar) because after 1582/10/04 is 1582/10/15. Note that it is also a smaller twin prime because 1582/10/11 (15821011) is also a prime and invalid date and if the leading zero in day is removed (1582109), that is also a prime number. [Marcel]