If you're born on January 1, you get the right answer all year long. If you're born on December 31, you get a new value on New Year's day and it'll be wrong all

If you're born on January 1, you get the right answer all year long. If you're born on December 31, you get a new value on New Year's day and it'll be wrong all year. EDIT: After re-reading you, yes, it's off-by-one for someone in the population for half the year on average.