Thought I'd put this here for safekeeping. Something I wrote in Dec 1999 for RogueChat Newsletter:
Thirty days hath November…April, June and September…All the rest have thirty-one, except February which has twenty-eight, except every four years when it has twenty-nine, except….Well, lets start closer to the beginning, shall we?
A year is 365.25 days long. So every four years those quarter-days add up to an extra day which we add to February, calling it Leap Year. For convenience and custom, it is “intercalated” to years evenly divisible by 4 (ie. 1968, 1980, 1996, etc.) Now that’s fine if it were exactly an extra quarter of a day per year.
Actually, a year is 365.242 days long. Every 100 years those leap year corrections have overcompensated, thereby adding one too many days. So century years, although divisible by 4, are NOT Leap Years. Most people, even computer programmers, know this. But that’s not exactly right. We’ve been adding not quite enough, because….
More accurately, a year is 365.2422 days long. We’ve been effectively dropping a day every 100 years to compensate for adding a little too much for Leap Years, which means we have not added quite enough. We add that day to centuries divisible by 400 (1600, 2000, 2400) thereby making the year 2000 a Leap Year. Many people, including calendar makers and computer programmers didn’t know this little catch, and there are tons of incorrect calendars gathering dust and countless bogus programs.
So the general rule is: Add a Leap Year day every four years except century years except century years divisible by 400 except…But first a word from our sponsor.
This has been my first opportunity to use the new Encyclopedia Britannica Online as an actual research source and I must say I’m very impressed. Mouse-clicking is a lot easier than dragging “dead tree” volumes off the shelf, cross-referencing and book-marking passages and scribbling notes and marking refs and…and you know how that goes. With Britannica Online, each entry includes enough links and sublinks to get a veteran web-surfer good and lost without even leaving the website. And, unlike the paper version, it has room to include interesting two-page articles related to the topic. A great example of which is this article by Robert Garland
which points out that, “Our present calendar has become so accurate that it will take 44,000 years before it falls out of step with the sun by so much as a single day.” Britannica Online should be an important addition to your virtual reference desk.
< Encyclopedia Britannica
Allow me to digress from this digression. For those of us who are compulsive clock-watchers, odometer-scrutinizers and closet numerologists, we just missed an important milestone in calendar coincidences.
11/19/1999 was an Odd day (all digits odd). This is the last one for a long, long while. After this, we won't see an odd day until 1/1/3111. The next even day will be 2/2/2000 (all digits even), the first since
Now, back to our regularly scheduled article.
Rules of intercalation: Every fourth year except century years except centuries divisible by 400. Which would be accurate if a year were precisely 365.2422 days long. But it’s not. The solar year (tropical year) is 365 days 5 hours 48 minutes 46 seconds long, and is decreasing slightly as the Earth's rotation accelerates, losing about 1.6 milliseconds per century.
Right now, a year is 365.242196 days long. So by overcompensating every 400 years for the century correction of the Leap Year addition, we leave ourselves just about a day long every 4000 years. That means that, even though the year 2000 is a Leap Year, the year 4000 is not. I hope you will remember that when you are laying out a calendar or programming a computer two millennia from now.
The accuracy of our timekeeping has advanced well beyond celestial observations. Using atomic clocks and computers we can even adjust for leap seconds every year or so. There is one little catch to using digital technology for keeping track of time, as pointed out in this Y2K Sonnet
Though time, whose majesty holds us in thrall,
Progresses like an arrow. Understood
By those who are aware. Although we brood,
It draws the margins of each life. And all
Who seek to live their lives must, yes, recall
The past, yet live the present --- as we should.
The future's just a veil of smoke, a pall.
In fact, it could be anybody's call.
Remember this for future history:
Though merely mortal sense is rarely keen,
It mocks the wit of literal machine
In sensing linearity of years.
Confusion, which reduces us to tears.
Machines don’t even know the century.