Really confusing.....how to solve this puzzle?

Two days before you met him was the 30th December 1976, and the boy was 10 years old. The next year after 1977 is 1978.This means that his birthday has to be after 30th December, but before the start of the new year. The only valid date is therefore 31st December. If he turns 13 on 31st December 1978, then his birthday must be 31st December 1965. It's actually quite simple when you know how to work with the knowledge you're given.