One extremely common bug even among good writers - saying "Jack was in his 30th year" to mean "Jack was 30" (and so should be in his 31st year). At least, I thi

One extremely common bug even among good writers - saying "Jack was in his 30th year" to mean "Jack was 30" (and so should be in his 31st year). At least, I think of this as a bug, but perhaps it is accepted by general convention? I have not seen any discussion of it.