Solve this riddle?

Question

If you've seen it before, please let others rack their brains for a while...

Overheard at a holiday party:

"My my, you've grown. How old are you?"
"The day before yesterday, I was 18. Next year I will be 21."

How can this be possible?

Answer 1

The person was born on 12/31 and the conversation was on 1/1.

12/30 (day before yesterday) = 18
12/31 (yesterday) =19
This year (1/1) = will turn 20 on 12/31
The next year the person will turn 21