Q: I was 29 the day before yesterday and next year I will be 32. This is true only one day in a year. What day is my birthday?

A: December 31st is the birthday so on December 30: age = 29

Dec 31: turns 30

Jan 1 : the day it is ‘now’ in the question

will turn 31 at the end of the current year (it is far away as it is Jan 1 and the birthday is Dec 31)

therefore the person will be 32 at the end of the * next* calendar year