1492
From Tardis Wiki, the free Doctor Who reference
In 1492, Christopher Columbus embarked on his first voyage to the Americas. (DW: The Masque of Mandragora) Sometime before, during, or after this voyage, the Doctor acquired the explorer's business card (DW: The Two Doctors)
- This date is not made explicit in Masque, but it is implied because Columbus' return in 1493 is specifically denoted.