Illinois State University mathematician Phil Grizzard points out that a person born on Nov. 30, 1999, is a “stopwatch baby” — the date always displays her age in months, days, and years. For example, today, 5/4/12, such a person has been alive for 5 months, 4 days, and 12 years. (Europeans can swap the month and day — the principle still works.)

A caveat: In December we must “make change” by setting the month to 0 and adding 1 to the year. So this Christmas, 12/25/12, a stopwatch baby will be 0/25/13 — 0 months, 25 days, and 13 years old.