(WHTM) — In 45 B.C.E. Rome adopted a new calendar proposed by Julius Caesar which, not surprisingly, became known as the Julian Calendar. (Credit where credit is due: a Greek astronomer and mathematician named Sosigenes did most of the calculations.)

In 1582, Pope Gregory XIII decided it was well past time to tinker with the Julian Calendar.

The problem was that the Julian Calendar had a minor flaw, which became a major flaw with the passage of time. Knowing (roughly) that a solar year was about 365.25 days, it introduced a correction — the leap year. You would have three years of 365 days, followed by a leap year of 366 days.

This was far more accurate than that which had gone before. For one thing, it actually made it possible to assign specific dates to solstices and equinoxes. But a leap year every four years slightly overcompensated for the actual length of a solar year — 365 days, 5 hours, 48 minutes, and 46 seconds. As a result, over 128 years, an error accumulated of about one day. By the mid-1580s, that error had built up to ten days and counting.

For Christians, this error turned the calculation of moveable feast days into a nightmare. This was especially true of Easter, which was set as the first Sunday after the first full Moon occurring on or after the spring equinox.

Gregory’s reform made a lot of changes, but the most important was how we reckon leap years. We’ll just let the U.S. Naval Observatory explain this one:

“Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the year 2000 is.”

 United States Naval Observatory

Pope Gregory instituted This reform by the papal bull “Inter gravissimas” on Feb. 24, 1582. The change went into effect in October, when Thursday, Oct. 4 was followed by Friday, Oct. 15.

But not everyone rushed to adopt the new calendar. Eastern Orthodox, Oriental Orthodox, and Protestant churches refused to recognize it. Protestants in particular feared calendar reform was a nefarious plot to drag them back to Catholicism. (Conspiracy theories are nothing new.) But slowly, as the advantages of the new calendar became clear, more European countries adopted it.

England held out against it until 1752. By then, the Julian calendar had added another day’s error, so the English were 11 days out of sync with the rest of Europe. This added a whole new level of aggravation to communication and trade with other countries.

Finally, on May 13, 1751, (Julian) the British House of Commons passed “An Act for regulating the Commencement of the Year, and for correcting the Calendar now in Use.” It received royal assent on May 22, and in 1752, England dropped 11 days from the calendar, hopping directly from Sept. 2 to Sept. 14.

Of course in 1752, “English” included the United States, which at the time were neither states nor particularly united. (They bickered amongst themselves rather a lot.) The change in the calendar meant people changed their birthdates; George Washinton was born on Feb. 11, 1731, but in the days before we lumped his birthday in with Lincoln’s birthday to create Presidents Day we celebrated Washington’s Birthday on Feb. 22.

Over time, the Gregorian calendar was adopted by countries around the world — Japan in 1873, China in 1912, and Saudi Arabia in 2016, just to name a few. It is now the closest thing we have on this planet to a universal calendar.

And now, with supermegahyperaccurate atomic clocks, we occasionally compensate for minuscule fluctuations in the Earth’s rotation by adding a leap second to the calendar. (We’ve done this 27 times since 1972.) This helps keep International Atomic Time (TAI), the time as measured by atomic clocks, synced up with Observed Solar Time (UTI) so that Coordinated Universal Time (UTC), the system we use for most of our day-to-day timekeeping, doesn’t stray too far from solar time — with an accuracy Julius Caesar and Pope Gregory could never have imagined.