Personally I love it when people do this. It's just.. it saves me the few seconds it takes, every time, to subtract "1" from the number.
It's not that hard with "20th century" which I'm conditioned to map to "last century" immediately, but when someone did something in the 17th century I first think "Oh like 1750-ish" and then I realize I'm a century off, and it disrupts my reading flow.