there is an american law that states a person is 18 years of age when they are 17 years and 364 days. this is because you're born on day 1 of your life, not day zero. it was case law from the civil war when a boy was drafted and argued he was too young. they ruled that a person reaches 18 years of age (legally different from 18 years old) the day before they're 18th birthday.
United States