You probably haven’t thought about this much before, but the calendar on your wall is, as a piece of civic infrastructure, bizarre. It’s the result of two and a half millennia of duct tape and wood glue.
\nThe current global standard, the Gregorian calendar, is a 1582 Catholic correction to a 46 BCE Roman correction to a lunisolar system (a calendar that tries to track the moon’s phases and the solar year at the same time) that had been failing for several hundred years. Each “fix” was designed by people doing their best with the astronomy they had, and each “fix” was adopted on a timeline of political and religious convenience.
\nHope you are ready for a lesson, because the history is much weirder than the calendar lets on. It also makes the question of “could we reform this?” feel more likely, but that is for tomorrow’s post.
\nBefore Julius Caesar, the Roman calendar was a lunisolar system with 355 days in a standard year. Because 355 days is about ten days short of a solar year, the calendar drifted forward against the seasons rapidly. The fix was an intercalary month called Mercedonius. Intercalary just means “inserted,” an extra month wedged into the year to drag the calendar back into alignment with the seasons. It was supposed to go in every two years, added by a group of priests called the College of Pontifices.
\nThe Pontifices had to decide whether to insert Mercedonius in a given year. The decision was political. If they liked the consuls in office, they’d insert it, and the consuls got an extra month of office. If they didn’t, they’d skip it. During the civil wars of the late Republic, the system broke down completely. By 46 BCE, the Roman calendar had drifted nearly 90 days out of alignment with the seasons. Winter festivals were happening in autumn. Spring planting celebrations were happening in summer.
\nCaesar, after his Egyptian campaigns, came home and decided to fix it. He brought in an Alexandrian Greek astronomer named Sosigenes, who proposed abandoning the lunar cycle entirely and going to a pure solar calendar of 365.25 days, with a single leap day every four years.
\nTo reset the seasons, Caesar made 46 BCE last 445 days. There was the standard 355-day year, plus the regular intercalary Mercedonius, plus two additional intercalary months totaling 67 more days, all jammed between November and December. Roman historians later called it annus confusionis, the Year of Confusion.
\nOn January 1, 45 BCE, the Julian calendar took effect, and it ran more or less unchanged for sixteen and a half centuries.
\nThe Julian calendar’s bet was that the solar year is exactly 365 days and 6 hours (because of the one leap day every four years).
\nBut the mean tropical year—the actual time it takes the Earth to complete a full seasonal orbit—is approximately 365.2422 days. If you break that decimal down:
\nSo the solar year is actually 365 days, 5 hours, 48 minutes, and 46 seconds. The Julian calendar assumes exactly 6 hours (which is 5 hours, 59 minutes, and 60 seconds). Subtract the true solar year from the Julian assumption, and you find the Julian year is too long by exactly 11 minutes and 14 seconds.
\nThat doesn’t sound like much. Over 128 years it accumulates to a full day. Over 1,257 years, the span from the Council of Nicaea in 325 CE (where the Catholic Church anchored its calculation of Easter to the spring equinox) to the late sixteenth century, it accumulated to about 10 days.
\nBy 1582, this had become a real problem. Nicaea had fixed the spring equinox to March 21. By Pope Gregory XIII’s time, the physical equinox was happening on March 11. Easter, which is anchored to the equinox, was steadily drifting later into the year. Without a fix, Easter was going to walk forward into summer and the Catholic Church wasn’t going to let the ressurection of Jesus Christ drift all over the place. Time to fix the calendar!
\nThe Catholic Church commissioned a calendar reform led by an Italian physician and astronomer named Aloysius Lilius and, after his death, finished and defended by a German Jesuit named Christopher Clavius. Clavius’s defense of the new system, is one of those documents nobody reads but quietly underwrites the modern world.
\nThe reform did two things.
\nIt deleted ten days. Pope Gregory XIII’s papal bull Inter gravissimas declared that Thursday, October 4, 1582, would be followed directly by Friday, October 15, 1582. Five through fourteen simply did not exist. The days of the week continued unbroken, Thursday to Friday, but the dates jumped. This realigned the calendar with the physical equinox.
\nIt changed the leap year rule. Under the Julian system, every year divisible by 4 was a leap year. Gregory’s reform added an exception. Century years are not leap years, unless they are divisible by 400. So 1700, 1800, and 1900 were not leap years. 1600 and 2000 were. This drops three leap days every four hundred years, bringing the mean Gregorian year to 365.2425 days. Long by only 26.8 seconds per year, which is small enough that the next correction won’t be needed for over 3,000 years.
\nIt’s an elegant fix. It also went over about as well as you’d expect a Pope’s calendar reform to go in Protestant Europe.
\nCatholic countries adopted on the day Rome decreed. Spain, Portugal, Italy, Poland, France (with a December skip instead of October). Protestant Europe refused. England, by then deep in its Protestant identity, was not going to take a calendar reform from “the Antichrist in Rome.” For 170 years, traveling between England and France required adjusting your calendar by ten days, and after 1700, eleven days.
\nEngland finally gave in in 1752, when Wednesday, September 2, 1752, was followed by Thursday, September 14, 1752. The American colonies came along. There’s a famous story about street riots demanding “give us back our eleven days.” Most historians now think the riots were largely embellished from a satirical Hogarth painting, but the eleven-day jump was real.
\nRussia held out longest. The Russian Orthodox Church refused the Gregorian calendar straight through the late Russian Empire. It took the Bolshevik Revolution to finally switch civil time. Lenin decreed the change in early 1918, with Wednesday, January 31, 1918, followed by Thursday, February 14, 1918. Thirteen days deleted. This is why the “October Revolution” of 1917 actually happened on what we now call November 7. The Russian Orthodox Church still uses the Julian calendar for liturgical purposes, which is why Orthodox Christmas falls on January 7 in countries that converted civilly but not liturgically.
\nAnd then there’s Sweden. Sweden tried to be clever. In 1700, they decided to make the transition gradual. Drop every leap year between 1700 and 1740, no jumps. They successfully skipped leap day 1700. Then the Great Northern War broke out, the bureaucracy got distracted, and they forgot to skip leap years 1704 and 1708.
\nThis left Sweden on a unique calendar. One day ahead of Julian, ten days behind Gregorian, aligned with no one. King Charles XII, recognizing the system was a mess, decided to revert to Julian. To do this, Sweden added an extra leap day to 1712, creating February 30, 1712, the only documented February 30 in civil history.
\nSweden finally adopted Gregorian normally in 1753.
\nI’m spending a couple of days on calendars to illustrate it’s just a mess as with all our methods for representing time. The Gregorian calendar isn’t optimal. It is a sixteenth-century Catholic patch on a first-century-BCE Roman patch on an even earlier patch. What we use today is chronological kludge.
\nNone of this had to be this way. It is what it is because of historical accident: which rulers held power, which church each country answered to, and how long each was willing to take a calendar from Rome.
\nThis matters because it means the idea of replacing the Gregorian calendar isn’t crazy. The Gregorian calendar replaced the Julian which replaced the Roman which replaced a number of earlier systems. There is no reason in nature that the calendar in 2200 will look like the one in 2026.
\nSeveral serious people have tried. The League of Nations debated calendar reform in 1931. Eastman Kodak ran on a thirteen-month calendar for sixty-one years. There’s a current proposal, from a Johns Hopkins economist and an astrophysicist, to switch the world to a perpetual calendar where every date falls on the same day of the week, forever.
\nTomorrow: the calendar that almost was.
\n\n\n", "date_published": "2026-06-15T10:00:00-05:00", "url": "https://llbbl.blog/2026/06/15/day-all-the-calendars-are.html", "tags": ["History","Time","30daysoftime","Calendars"] } ] }I’d appreciate a follow. You can subscribe with your email below. The emails go out once a week, or you can find me on Mastodon at @logan@llbbl.blog.
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