Today is Leap Day, February 29, 2016. Leap years, under the Gregorian calendar used today, occur in conformity with three rules:

1. The year can be evenly divided by 4;
2. If the year can be evenly divided by 100, it is NOT a leap year, unless;
3. The year is also evenly divisible by 400, then it is a leap year.

Which begs the question, when do people born on leap day have their birthday in a common year (a common year is any year that is not a leap year).

Traditionally, where the common law prevails, one’s age is computed by including the day of birth, so that a given age is attained the day before the anniversary of your birthday. In other words, under the common law, a leap day baby appears to celebrate their birthday in a common year on February 28.

Missouri follows the common law from England. Section 1.010 RSMo., provides in part that:

1. The common law of England and all statutes and acts of parliament made prior to the fourth year of the reign of James the First, of a general nature, which are not local to that kingdom and not repugnant to or inconsistent with the Constitution of the United States, the constitution of this state, or the statute laws in force for the time being, are the rule of action and decision in this state, any custom or usage to the contrary notwithstanding, but no act of the general assembly or law of this state shall be held to be invalid, or limited in its scope or effect by the courts of this state, for the reason that it is in derogation of, or in conflict with, the common law, or with such statutes or acts of parliament; but all acts of the general assembly, or laws, shall be liberally construed, so as to effectuate the true intent and meaning thereof.

Under Missouri law, it is likely that the courts would say February 28 is the birthday of a leaper in a common year, but there is no decision that specifically reaches this result.

Other states follow the “birthday rule”, under which a person attains a given age on the actual anniversary date of his or her birth, not the day before. In states that follow the birthday rule, a leap day baby would celebrate his or hear birthday on March 1 in a common year.

So when do people born on leap day have their birthday in a common year? The short answer appears to be on whichever date they prefer, unless there is a specific statute in your state to address the issue.