An odd fact: Under the common law, as reported by Blackstone’s Commentaries:
[F]ull age in male or female is twenty one years, which age is completed on the day preceding the anniversary of a person’s birth.
And here is the explanation from Edward Christian, who edited the 1803 edition of Blackstone,
If he is born on the 1st of January, he is of age to do any legal act on the morning of the last day of December, though he may not have lived twenty-one years by nearly forty-eight hours: the reason assigned is, that in law there is no fraction of a day; and if the birth were on the first second of one day, and the act on the last second of the other, then twenty-one years would be complete; and in the law it is the same whether a thing is done upon one moment of the day or on another.
Not terribly persuasive to me, but there is. In any event, this turns out to have yielded a considerable amount of caselaw, most recently People v. Woolfolk (Mich. Ct. App. 2014), which rejects the common-law rule (even with the modern age of majority of 18 replacing the old 21), and which accepts a rule that your birthday is your birthday. The result: Deandre Woolfolk, who was born January 29, 1989, and who murdered Mone Little on January 28, 2007, is treated as not having turned 18 on that day, and thus gets the benefit of Miller v. Alabama, the Supreme Court case that bars mandatory life-without-parole sentences for murderers who were under 18 at the time of their actions.