Is it really the year we think it is?

First of all, we need to define our timescale. If we are using the Jewish, Islamic or other calendar, then no. On the other hand, if we are referring to the Gregorian calendar, then no also!

The problem is that there was no AD 0 (time moved smoothly from 1 BC to AD 1 with no intervening year). So, Christ’s first birthday would have been celebrated at the end of AD 1 and his tenth at the end of AD 10. It follows that the end of the year 1999 (also the beginning of the year 2000) marked 1999 years since the birth of Christ. Or, to put it another way, the year 2000 is the last year of the second millennium and not the first year of the third.

3 BC… --> …2 BC… --> …1 BC… --> …AD 1… --> …AD 2… --> …AD 3…

In fact, this is only one in a series of blunders through the ages that means we could be adrift by much more than one year.

Time present and Time past

It is thought that the first attempt at establishing a calendar based on the year of Christ’s birth was by a sixth-century abbot and mathematician, Dionysius Exiguus. He was charged by the Pope, in AD 525, with calculating the future dates of Easter. Some years later, the abbot produced a chart in which the first year was called anno Domini nostri Jesu Christi DXXXI—or AD 531. Unfortunately, it is quite likely that Dionysius Exiguus got this wrong.

No-one is certain exactly when Christ was born, but most biblical scholars take the view that it was probably in 4 or 5 BC. If this is right, then our calendar is already adrift by a number of years.

And we can't pile the blame on Dionysius — accuracy problems existed with calendars well before his time, indeed even before Christ’s birth.

Friends, Romans and countrymen, lend me your years!

Prior to the time of Julius Caesar, the Roman calendar had been based on the cycles of the moon and comprised 12 lunar months, or 354 days. The Romans compensated for the difference between the number of days in their calendar and the 365 days in a solar year by adding an extra 11 days here and there.

However, the result was still inaccurate by one quarter-day per year and, so, Caesar borrowed the idea of a leap year from the Egyptians and changed the basis of the calendar from lunar to solar. The Julian calendar was born.

Look before you leap

Because of the reach of the Roman Empire, the Julian calendar spread throughout Europe. Unfortunately, it was inherently inaccurate—the solar year is slightly less than 365¼ days long. The mean solar year is, to be precise, 365·242198778 days long (or 11 minutes 14 seconds less than 365¼ days).

At first, the difference was too small for anyone to notice and later, when it became more obvious, few were brave enough to openly challenge the established system. One exception was the British friar Roger Bacon who, in the 13th century, wrote to Pope Clement IV about the inaccuracies. But, it was some 300 years before the inaccuracy was addressed and, by then, the error had built up to 10 days. For example, the spring equinox was occurring on 11 March rather than 21 March.

The peasants are revolting

In 1582, Pope Gregory ordered that the 10-day deficit be corrected, restoring the calendar’s alignment with the sun, and modified the leap-year adjustment so that the alignment would be maintained in future years. This was achieved by treating those century years divisible by four hundred (e.g, 1600, 2000, etc) as normal leap years but other century years (1700, 1800, 1900, etc) as ordinary or common years. The modified calendar became known as the Gregorian calendar and is the one we use today.

The 10-day correction (the calendar skipped directly from October 4th to October 15th) was adopted by Italy, France Portugal and Spain in AD 1582. At the time, it caused a great deal of consternation — peasants revolted believing that ten days of their life had been taken away.

Within a few years, many other European countries had also adopted the new calendar; Scotland adopted it in 1600 but Germany resisted until about 1700. England, Wales and Ireland and the Colonies (including North America) held out until 1751 choosing to remove the then deficit of eleven days during September 1752. At the same time, it was declared that 1 January would mark the start of each year. One interesting point is that, in the following March, bankers refused to pay their taxes on the due date of 25 March, delaying by eleven days until 5 April. This explains why this date is still used today to mark the end of the British Tax Year.

Many of the remaining European states eventually adopted the Gregorian calendar and, via various empires, its use spread across the globe. However, progress was slow—for example, the calendar wasn't adopted by China until 1912 and Greece until 1923. Today, it is used throughout the Western world and parts of Asia.

Hold on a second

Although the Gregorian calendar provides a significant improvement in accuracy it isn’t perfect—it's about 26 seconds too slow per year. This error has already built up to some three hours and so, our clocks are adrift by this period.

And, yes, you've guessed it, all those 26-second errors will eventually build up into a whole, extra day. Present thinking is that it will be necessary to cancel a leap-day in either AD 4000 or 8000—which is not yet decided (it will depend upon the minor variations in the Sun–Earth relationship).

Next please...

Whilst the current calendar is a great improvement on those that have gone before, it still has a number of annoying inconsistencies. For example, the months, the quarter-years and the half-years are of unequal length. There have been many proposals to simplify it.

One proposal, gaining some popularity, is to split the year into four-quarters of thirteen weeks each and for each day of the year to be assigned a fixed day of the week. The mathematicians amongst you will have recognised that 4 × 13 × 7 equals 364, one day too short for a normal year. Consequently, an additional ‘blank day’ will be required each year, in addition to an extra day in leap years.

Interestingly, under this proposal, Easter is likely to always fall on the same day, possibly Sunday 8 April. Which brings us neatly back to where we started.

Try as they may to savior the taste of eternity,
their thoughts still twist and turn upon
the ebb and flow of things in past and future time.
But if only their minds could be seized and held steady,
they would be still for a while and, for that short moment,
they would glimpse the splendour of eternity, which is for ever still

—Augustine of Hippo, about AD 400

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Page updated 08/08/2005