Ok, let's say the date is 1 Jan 2016.
The day before yesterday he was 8 (Yesterday = 31 Dec 2015 and the day before was 30 Dec 2015).
His birthday was the 31 Dec 2015 when he was 9. On the 31 Dec 2016 he is 10 and on the 31 Dec 2016 he is 11.
Brain Teaser Time Again...
The day before yesterday, Young Jim was 8 years old. Next year, he'll turn 11. How is this possible?
You would fricken love this show.