When do leap-day babies celebrate their birthdays?

Dear Cecil:

I have always wondered what people do who are born on leap day, February 29. Obviously they age each year, but do they celebrate it on the 28th or the 1st? And when their actual birth date does come around, do they have a really huge bash to make up for lost time?

Cecil replies:

Dear Kristin:

My assistant Little Ed explained this in his book Know It All, so you know it can’t be that complicated. What you celebrate on your birthday isn’t the annual arrival of your birth date; it’s the fact that you’re one year older. One year = one complete revolution by the earth around the sun = 365 days, 5 hours, 48 minutes, and 46 seconds.

To figure the right day to celebrate your birthday, you add 365 and one-quarter days to the hour of your birth. Suppose you were born February 29, 1972 at 10 PM. Then 365 and one-quarter days went by and behold, the first anniversary of your birth hour came on March 1, 1973, at about 4 AM.

The second and third anniversaries also fell on March 1, at 10 AM and 4 PM respectively. Comes year four (1976), and your anniversary is back where it started, February 29 at 10 PM.

Things would have worked out differently if you’d been born at 4 AM on leap day. Your first, second, and third birth-hour anniversaries would have occurred on February 28 at 10 AM, 4 PM, and 10 PM, respectively. If you’d been born at 4 PM, your first anniversary would fall on February 28 but your second and third on March 1. What happens for leap-day babies born at other hours is left as an exercise for the student.

The real problem isn’t leap-day people, it’s those smug non-leap-day babies who think all they’ve got to do to be in synch with the cosmos is celebrate their birthdays on the same date every year. Not a chance.

If you were born February 28, 1972, at 4 AM, you were supposed to celebrate all your non-leap-year birthdays on February 27. Did you? Of course not. Before you were out of diapers you were shaking down the ‘rents for gifts under false pretenses. Considering how today’s youth start out, it’s no wonder so many come to no good.

But look on the bright side. The year 2000 will be a normal leap year. Years divisible by 100 usually aren’t. (The rule is: year divisible by 100, no leap year unless also divisible by 400, in which case leap year. The idea is to keep the calendar lined up with the solar system. Trust me.)

Were we to skip a leap year in 2000, the awful consequence would be that everybody in the world would celebrate his or her birthday on the wrong day. (At least in some years. To get technical, on average we’d be 66 percent more wrong than previously.) Talk about dodging a bullet.

In leap-day-less 1900 they weren’t so lucky. Take my late grandmother, born in 1887. Commencing in 1900 she began celebrating her birthday a day before it actually occurred. For the next 81 years, in short, she was living a lie. She was a dour woman; no doubt that’s why.

Send questions to Cecil via cecil@straightdope.com.

Comment on this Column