A Brief Word on Dates

If you ever decide to do some looking into your family history, and you make it back to, say, the early 1700s, you will probably see one or more dates written like this: Jan 7, 1719/20. Unlike what you might think, that doesn’t mean the author was unsure of what year the event happened in, so they wrote both; rather, it indicates the event occurred on Jan 7 of… either 1719 or 1720, depending on whether you go by the contemporary calendar of the time, or our modern calendar today. And, making matters worse, it was not a Jan 7 that was an even number of years removed from this year’s Jan 7. Worst of all, it may also have reflected a culture that didn’t acknowledge the new year until the end of March. March?

I know, “clear as mud.”

If you’re scratching your head at all this, the problem stems from a major disruption in the measurement of time that occurred when Europe, and Europe’s colonies, switched – slowly, over several years – from the calendar that had been employed by the Romans, aka the Julian calendar, to an updated system, adopted by Pope Gregory, aka the Gregorian calendar.

This is not a minor point.

It affects the citation of all “early” colonial historical dates in a meaningful way, and the explanation below is, I’m sorry to say, actually worth wading through…



The Julian and the Gregorian Calendars

by Peter Meyer


The Gregorian Reform

The average length of a year in the Julian Calendar [the calendar put in place by Julius Caesar]  is 365.25 days (one additional day being added every four years). The length of the year in the Julian Calendar exceeds the length of the mean solar year (365.24219 mean solar days to five decimal places) by 11.2 minutes. This error accumulates so that after 128 years the calendar is out of sync with the equinoxes and solstices by one day. Thus as the centuries passed the Julian Calendar became increasingly inaccurate with respect to the seasons. This was especially troubling to the Christian Church because it affected the determination of the date of Easter, which, by the 16th Century, was well on the way to slipping into Summer.

Pope Paul III recruited several astronomers to come up with a solution, principally the Jesuit Christopher Clavius (1537-1612) who built upon calendar reform proposals by the astronomer and physician Luigi Lilio (d. 1576). When Pope Gregory XIII was elected he found various proposals for calendar reform before him, and decided in favor of that of Clavius. On 1582-02-24 he issued a papal bull, Inter Gravissimas, establishing what is now called the Gregorian Calendar reform. The Gregorian Calendar is the calendar which is currently in use in all Western and Westernized countries.

The Gregorian reform consisted of the following: Ten days were omitted from the calendar, and it was decreed that the day following (Thursday) October 4, 1582 (which is October 5, 1582, in the old calendar) would thenceforth be known as (Friday) October 15, 1582. The rule for leap years was changed. In the Julian Calendar a year is a leap year if it is divisible by 4. In the Gregorian Calendar a year is a leap year if (a) it is divisible by 4 and (b) if it is divisible by 100 then it is divisible by 400. In other words, a year which is divisible by 4 is a leap year unless it is divisible by 100 but not by 400 (in which case it is not a leap year). Thus the years 1600 and 2000 are leap years, but 1700, 1800, 1900 and 2100 are not. New rules for the determination of the date of Easter were adopted (the old rule relied on the Jewish calendar). The first day of the year (New Years Day) was set at January 1st. The position of the extra day in a leap year was moved from the day before February 25th to the day following February 28th. The term “leap year” derives from the fact that the day of the week on which certain festivals were held normally advanced by one day (since 365 = 7*52 + 1), but in years with an extra day the festivals would “leap” to the weekday following that.

It may be noted that there was no necessity for ten days, rather than, say, twelve days to have been omitted from the calendar. In fact, the calendar could have been reformed without omitting any days at all, since only the new rule for leap years is required to keep the calendar synchronized with the vernal equinoxes. The number of days omitted determines the date for the Spring equinox, an omission of ten days resulting in a date usually of March 20th.

The average length of the vernal equinox year during the last 2000 years is 365.242 days. The average length of the Julian year (365.25 days) differs from this value by 0.008 days. So from the year 1 to the year 1582 the calendar drifted off the mean solar year by 1581*0.008 = 12.6 days. Why didn’t Pope Gregory remove twelve days, instead of just ten? It has to do with the First Council of Nicea, which was held in Nicea (now Iznik, Turkey) in the year 325. One of the matters settled by this council was the method for determining the date of Easter. From the 325 to 1582 the calendar diverged by 1257*0.008 = 10.1 days, so ten days were removed so as to restore the date of Easter to the same time of the year at which it had occurred at the time of the Council of Nicea.

(iii) Adoption of the Gregorian Calendar

The Gregorian Calendar was adopted immediately upon the promulgation of Pope Gregory’s decree in the Catholic countries of Italy, Spain, Portugal and Poland, and shortly thereafter in France and Luxembourg. During the next two years most Catholic regions of Germany, Belgium, Switzerland and the Netherlands came on board. Hungary followed in 1587. The rest of the Netherlands, Denmark, Germany and Switzerland made the change during 1699 to 1701.

By the time the British were ready to go along with the rest of Europe, the old calendar had drifted off by one more day, requiring a correction of eleven days, rather than ten, to locate the Spring equinox at March 21. The Gregorian Calendar was adopted in Britain (and in the British colonies) in 1752, with September 2, 1752, being followed immediately by September 14, 1752.

In many countries the Julian Calendar was used by the general population long after the official introduction of the Gregorian Calendar. Thus events were recorded in the 16th – 18th Centuries with various dates, depending on which calendar was used. Dates recorded in the Julian Calendar were marked “O.S.” for “Old Style”, and those in the Gregorian Calendar were marked “N.S.” for “New Style”.

To complicate matters further New Year’s Day, the first day of the new year, was celebrated in different countries, and sometimes by different groups of people within the same country, on either January 1, March 1, March 25 or December 25. January 1 (decreed by Pope Gregory) seems to have been the usual date but there was no standard observed. With the introduction of the Gregorian Calendar in Britain and the colonies New Year’s Day was generally observed on January 1. Previously in the colonies it was common for March 24 of one year to be followed by March 25 of the next year. This explains why, with the calendrical reform, the year of George Washington’s birth changed from 1731 to 1732. In the Julian Calendar he was born on 2/11/1731 but in the Gregorian Calendar his birthdate is 2/22/1732.

Sweden adopted the Gregorian calendar in 1753, Japan in 1873, Egypt in 1875, Eastern Europe during 1912 to 1919 and Turkey in 1927. Following the Bolshevik Revolution in Russia it was decreed that thirteen days would be omitted from the calendar, the day following January 31, 1918, O.S. becoming February 14, 1918, N.S. Further information can be found in The Perpetual Calendar.

In 1923 the Eastern Orthodox Churches adopted a modified form of the Gregorian Calendar in an attempt to render the calendar more accurate (see below). October 1, 1923, in the Julian Calendar became October 14, 1923, in the Eastern Orthodox calendar. The date of Easter is determined by reference to modern lunar astronomy (in contrast to the more approximate lunar model of the Gregorian system).


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