By Steve Hendrix

The Washington Post

We’re all Gregorians now. But it took a while.

On New Year’s Day, just about every corner of the world flips a calendar in accordance with the system commissioned by Pope Gregory XIII in the 1580s. Contracts, leases and birthday candle production all depend on the Gregorian agreement that the year begins on Jan. 1 and lasts exactly 365 days, five hours, 49 minutes and 12 seconds. Even in cultures that maintain their traditional calendars as well, the smartphones will all tick over to 2018 on the same day.

But it took some of us longer than others to fall into line behind the good Gregory’s celestial timekeeping. The British, and their American colonies, did not like the idea of having their dates dictated by the Catholic Church and so stuck with the previous calendar makeover, the one done by Julius Caesar in 45 B.C. They waited until 1752, by which time the more wobbly Julian calendar was off track by 11 whole days.

That year, to line up with the Gregorian system, the Brits and Americans lopped 11 days out of September in one chunk. At one second after midnight on Sept. 2 it became Sept 14. Everyone’s birthday shifted accordingly. George Washington, who was born Feb. 11, 1732, celebrated his 20th — and each subsequent one — on Feb. 22, a change that mattress and furniture stores observe to this day.

“There were riots in Britain that year,” said Denis Feeney, a classics professor at Princeton University. “People wanted their 11 days back. It all comes down to whether you are looking at the day or the date.”

Calendar management — which is essentially applying a man-made template over the movement of the Earth relative to the sun or the moon — has been a maintenance chore for humanity since the dawn of, well, dawns. (We may need to do it again around the year 4,000, so don’t make any appointments beyond then just yet.)

Every culture had its way, from the Mayan to the Mesopotamian. The Western tradition started, according to custom, around 509 B.C. when Rome’s second king instituted the Roman Republican Calendar.

That 355-day system was based on the phases of the moon and featured 12 months, one of which lined up with the winter solstice and was named for Janus, the two-faced Roman god of endings and beginnings. The 12 Roman months are familiar still. (The fact that the number names — September through December — do not line up with their number month probably reflects the fact that the official Roman year started in March, when the consuls met and war fighting season began).

The Republican Calendar, which lasted nearly half a millennium, featured nine-day weeks with years that were most often named for the ruling consul (the numbering of years did not start until centuries later). But it needed frequent tweaking to pull it back into the alignment with the seasons, a job that fell to appointed officials who would determine how long of a “leap month” to add after each February. One was a young leader named Julius Caesar, who had grown so frustrated with the wandering day count by the time he became emperor that he decided to scrap the entire system and remake it in his own name.

“It was a political decision as much as anything,” said Sacha Stern, author of “Calendars in Antiquity” and a professor at University College London. “Julius Caesar wanted to make his mark on society.”

Caesar brought in an Alexandrian astronomer to adapt the Egyptian system, which was based on the solar year. Using their best calculations, they extended the year to 365.25 days long and added at least 67 days to that year, bringing Jan. 1, 45 B.C. in line with the start of winter. And finally, to make the year as self-regulating as possible, they built in a leap day to recur every four years after Feb. 24. The month of Quinctilis was renamed “July” in honor of Julius Caesar, and the new calendar ticked into life.

Effectively, this was the beginning of verifiable time in the West. Historians can fix nearly exactly any event that occurred after Jan. 1, 45 B.C., after some adjustment calculations. Anything before demands a lot of guesswork.

Feeney points out that one of the first certain dates in Roman history is Caesar’s own assassination on the 15th of March, 44 B.C. Of the birth of Cicero, on the other hand, given as Jan. 3, 106 B.C., he said, “We cannot pin that down with nearly as much precision.”

As the Julian calendar marked the passing of century after century, its shortcoming became clearer. The years were about 11 minutes too long, thus losing about three days every 400 years. That was not slippage that anyone would notice in a lifetime, but by the 16th century, it had added up. The dates were about 10 days misaligned with solstices, church festivals and other seasonal markers. Easter in particular (which depends on it own lunar formula) was way out of whack. In 1582, Pope Gregory XIII decided that a tweak was due.

It was not a big change. His Jesuit astronomer dropped 10 days from that year to get things reset and corrected future slippage by eliminating three leap days every 400 years. We still follow Gregory’s rule: There is no leap day (Feb. 29) in centennial years (1700, 1800 and 1900) except for centennial years divisible by 400 (which is why 2000 was a leap year but 2100 will not be).

Got that? Don’t worry. The astronomers and makers of cute kitten calendars are keeping a close eye on it for you. Some researchers have suggested that another tweak will be due by 4000 A.D., when the Gregorian system will have gone off by about a day. But for the next 1,982 New Years, make any plans you like.

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