Leap Days and Leap Years

When to celebrate your birthday?

It’s common knowledge that “leaplings” or “leap year babies” have a problem deciding when to celebrate their birthday. They are born on 29 February, so every 4th year they celebrate their birthday on 29th February. But what to do in the years between? Some celebrate on February 28th, others do it on March 1st, and some decide to not celebrate at all; they skip 3 birthdays in a row to which they are astronomically entitled.

You are entitled to a birthday every time when you have completed one trip around the sun.

Not only leaplings have a birthday problem

Actually, “the problem when to celebrate a birthday” is not limited to leaplings. Many people celebrate their birthday on the wrong day, without even being aware of it. Probably even you. Why is that?

A birthday is meant to be celebrated the moment that you are one year old; or two years, or three, or seventy five. But a year as we know it (365 days) is not a real year. A year is actually a bit longer; an astronomical year (the period of the earth moving around the sun) is about 365.25 days long. That’s the reason we have invented leap years.

But this means that you should celebrate your first birthday at 365.25 days after you are born. And your second birthday at 730.5 days. Let’s see what this means.

For the record: I am born on 3rd February 1957 at 5:00 AM. We will see that that was very convenient.

If you are born on 3rd February 1957 at 5:00 AM, you will celebrate your first year on earth (first birthday) on 3rd February 1958 at 11:00 AM. You are 2 years old on 3rd February 1959 at 17:00. You are 3 years old on 3rd February 1960 at 23:00. And conveniently you will be 4 years old on 3rd February in 1961 at 5:00 AM (thanks to the 1960 leap year). Great. I can celebrate all my birthdays on 3rd February.

03/02/1957 05:00
03/02/1958 11:00
03/02/1959 17:00
03/02/1960 23:00
03/02/1961 05:00
03/02/1962 11:00
03/02/1963 17:00
03/02/1964 23:00
03/02/1965 05:00
03/02/1966 11:00
03/02/1967 17:00
03/02/1968 23:00

But what if you were born a month later than me, on 3rd March 1957 at 5:00 AM? You will be one year old on 3 March 1958 at 11:00 AM. You will be two on 3 March 1959 at 17:00. But 1960 is a leap year (29 days in February), so you will be 3 years old on 2nd March 1960 at 23:00. That seems to be a day early but it isn’t. Then the next year you can celebrate again on March 3rd, because you will be 4 years old on 3-3-1961 at 5:00 AM. But the point is: once every 4 years you will need to celebrate on 2nd March instead of 3rd March.

03/03/1957 05:00
03/03/1958 11:00
03/03/1959 17:00
02/03/1960 23:00
03/03/1961 05:00
03/03/1962 11:00
03/03/1963 17:00
02/03/1964 23:00
03/03/1965 05:00
03/03/1966 11:00
03/03/1967 17:00
02/03/1968 23:00

But wait a minute. What if you were born just 2 hours after me, on 3rd February 1957 at 7:00 AM? You will be one year old on 3 Feb 1958 at 13:00. You will be two on 3 Feb 1959 at 19:00. And you will be three years old on 4 February 1960 at 01:00 AM. In 1961 your 4th year on earth will be completed on 3 Feb again, at 7:00 AM. But once every 4 years you have a birthday on 4th February instead of 3rd February.

03/02/1957 07:00
03/02/1958 13:00
03/02/1959 19:00
04/02/1960 01:00
03/02/1961 07:00
03/02/1962 13:00
03/02/1963 19:00
04/02/1964 01:00
03/02/1965 07:00
03/02/1966 13:00
03/02/1967 19:00
04/02/1968 01:00

See how lucky I am to celebrate all my birthdays on my birthday? It’s a matter of hours.

Now what about these leaplings? When do they have to celebrate their birthdays? It’s not that easy, because it depends on the time they were born. Leaplings born before 6.00 AM will celebrate on 28 February, except in leap years when they celebrate on 29th.

Leaplings born after 18:00 PM will celebrate on 1st March, except in leap years when they celebrate on 29 February.

And leaplings born between 6:00 AM and 18:00 PM will have to celebrate their birthdays moments on 3 different dates as you can see in the following table (for somebody born on leap day 1980 at 13:00 PM):

29/02/1980 13:00
28/02/1981 19:00
01/03/1982 01:00
01/03/1983 07:00
29/02/1984 13:00
28/02/1985 19:00
01/03/1986 01:00
01/03/1987 07:00
29/02/1988 13:00
28/02/1989 19:00
01/03/1990 01:00
01/03/1991 07:00
29/02/1992 13:00

What about you?

Of all the people in the world, more that 75% (actually 75.02%) would have to celebrate their birthday on a different day once in a while. I call this a “leap birthday”. Only 24.98% of all people are born conveniently on such a moment that they can celebrate their birthday each year on the same date. What about you? When will you have to celebrate your birthdays?

To make it easy, I have written a PHP script that can create your birthday calender. Enter day, month, year and time of birth, then press the “calculate” button. To use this calculator you need to know only roughly at what time you were born. It’s the quarter of the day that make the difference (00:00 – 06:00, 06:00 – 12:00, 12:00 – 18:00 and 18:00 – 24:00). Note that a pair of twins could end up celebrating their birthday once in a while on different days if they were born several minutes apart but in a different quarter of the day.

Click here to calculate your birthdays.

Birthday cake
Happy Birthday

Did you know this?

The length of a year is also not exactly 365.25 days. It’s closer to 365.2425 days. That means that having a leap year every 4th year is not exactly right.

The rules for leap years are therefore:

A year is a leap year if it can be divided by 4. But if it can be divided by 100 it is not a leap year. However, if it can be divided by 400, then it is a leap year.

So 1700, 1800, 1900 and 2100 are not a leap year, but 2000 is a leap year.

Funny is that somebody who is born just before 1900 or just before 2100 will have a shift in birthdays at the turn of the century. Check this in the script. Try somebody born 2 Sept 2092 in the first quarter of the day.

See also: Proverbs and quotes about leap years and leap days.

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