DATES AND DATING: JULIAN AND GREGORIAN CALENDARS

When you’re dealing with events that took place four or five hundred years ago, the exact dates when those events occurred are not always obvious—even if dates are provided in a primary-source document from the time.

The problem is calendars.

Until the autumn of 1582, all the various European countries used the same calendar: the Julian calendar, named after Julius Caesar, who was responsible for its introduction. In 1582, however, Pope Gregory XIII introduced a new calendar: the Gregorian calendar—named after him. This calendar change can make keeping track of sixteenth, seventeenth, and even some eighteenth century dates a little tricky.

To understand why there were two calendars, why one replaced the other, and how the dating in the two systems differs, some background explanation is necessary.

There’s a basic problem with calendars. They are human creations: carefully regularized, systematic constructs. They contain so many hours per day, so many days per week, so many weeks per month, so many months per year… repeating year after year after year. The universe, however, is not that perfectly regularized.

People traditionally marked the cycle of the year by reference to celestial events—solstices, equinoxes, phases of the moon—or to the seasons. These natural events punctuate the year and have their own consistent timing, but that timing does not necessarily translate into exact units of hours/days/weeks/months.

Until the time of Julius Caesar, the Romans struggled with their calendar. They were constantly tweaking it—adding or subtracting days—for religious, political, military, and practical reasons. As a result, as the centuries passed, the calendar drifted, and the link between the natural events that punctuated the year and the calendar dates that corresponded to those events was lost.

Julius Caesar eliminated this confusion by arbitrarily re-synchronizing the dates and establishing a new calendar. This calendar—the Julian calendar—was fundamentally the same as the one we are familiar with today: a twelve‑month year equaling 365 days, the months having either 30 or 31 days, except for February which has 28, with a leap year every fourth year during which an extra day is added to February. Even the names of our months derive from the Julian calendar (like September, October, November, December, based on the Latin words for seven, eight, nine, and ten).

This re-synchronized, regularized calendar was a huge improvement over what had come before, and it was used throughout the Roman Republic/Empire and its various Europe progenies for a millennium and a half.

There was, however, a problem.

The Julian system had a regular, unchanging four-year cycle: three years of 365 days followed by one year of 366 days. This means that each year in the four‑year cycle averages out to be 365¼ days long. The problem is that the actual time it takes the Earth to make one full orbit around the sun is slightly less than 365¼ days. Depending on how one takes the measurement, the time required for the Earth to circle the sun turns out to be between 365.2422 and 365.2424 days. This may not seem like much of a difference, but it means that the length of a full Julian-calendar year was about 11 minutes (between 10.94 and 11.23 minutes, depending on how one measures it) longer than the actual length of a year (i.e., than the actual time it takes the Earth to complete exactly one orbit around the Sun).

This small annual creep adds up to about one full 24‑hour day every 128 years. By the middle of the sixteenth century (after roughly a millennium and a half of use), the Julian calendar was about ten days ahead of the natural events that punctuated the year. The typical date for the spring equinox, for example, would have been March 30 rather than March 20.

There is a counter-intuitive element to all of this.

The Julian‑calendar year is slightly too long, so the calendar is sometimes described as being slow.

The consequence of this slowness was that, as the centuries passed, the Julian‑calendar dates got ahead of the natural events that punctuated the year.

Think of it this way. You have two watches: watch A (corresponding to events in the natural word), which takes precisely 24 hours to clock a 24‑hour period, and watch B (corresponding to the Julian calendar), which takes 24 hours and 1 second to clock that same 24‑hour period. In other words, watch B is running slowly, and by the time watch B clocks 24 hours, watch A will have clocked 24 hours and 1 second.

Now think of synchronizing these two watches and starting them simultaneously on an imaginary Day 1. The two watches would start out the same, but midnight (12:00:00 AM) at the end of Day 1 for watch B would be 12:00:01 AM, Day 2, for watch A; Day 3 would be 1200:00:02 AM, and so on. Now imagine that an event happens at noon on Day 10. If the bearer of watch A showed up precisely at noon according to her watch, she would be there at 12:00:00. But if the bearer of watch B showed up precisely at noon according to her watch, she would be there at 12:00:10 according to watch A.

Now imagine this event is an annual one. By the next year, the bearer of watch A would still clock the noon event as 12:00:00 and be there precisely on time. The bearer of watch B would show up at noon as well, but 12:00:00 for watch B would now be 12:06:05 for watch A. Multiply this kind of creep by decades, centuries, and millennia, and you have the problem with the Julian calendar.

By the third quarter of the sixteenth century, this calendrical creep was creating practical difficulties for the Catholic Church. Calculating dates for Easter, for example, and having those dates connect appropriately with other Church holy days, had become progressively more and more complicated.

A great deal of discussion and dispute led up to the eventual solution.

On February 24, 1582 (Julian), Pope Gregory XIII issued a papal bull commonly known as the Inter Gravissimas (from the opening words of the original Latin text: Inter gravissimas pastoralis officii nostri, “Among our serious pastoral duties”) in which he decreed that 10 days be dropped from the calendar. As a result, Thursday October 4 (Julian) 1582 was immediately followed by Friday October 15 (Gregorian) 1582. In other words, 10 days—October 5 through 14 (Julian) inclusive—were removed and the calendar date skipped ahead, but the continuity of the days of the week remained unchanged. As a result, the calendar dates were re-synched with the natural events punctuating the year.

More was needed than just the deleting on those ten days, though. That would have been like re-setting watch B but letting it continue to run slowly; the calendrical creep would just have built up again.

In the Inter Gravissimas, Pope Gregory XIII also laid out a formula for curtailing leap years so that his new calendar would not drift away from the natural events punctuating the year in the way the Julian calendar had. The solution was this: beginning century years (1600, 1700, 1800, and so on) would only be leap years if they were divisible by 400. This restriction eliminates some leap years and so reduces the overall calendrical creep to something like one day in every 3,333 years, enough that the Pope could claim his new calendar might be used in perpetuum to reliably establish holy days.

The new Gregorian calendar was adopted quickly by Catholic countries. Protestants, however, did not cooperate. This means that Catholics and Protestants frequently—but not always—used different dating systems throughout the last quarter of the sixteenth and the seventeenth centuries and, in some cases (like England), well into the eighteenth.

There’s nothing especially complicated about Julian or Gregorian calendars, and converting dates from one system to the other isn’t hard (just add or subtract 10 days). However, if you’re reading seventeenth century documents and don’t pay attention to which calendar system is being used, you can get confused about chronology.

And if you’re confused enough, you can end up drawing wrong conclusion about timing and about the cause/effect of events. So it’s important to get the Julian and Gregorian dates clear.

Just one more example of the old saying…

The devil is in the details.