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Home > History > The Bible in History > Calendar Systems > Part 5 - Modern Gregorian Calendar

Part 5 - Development of our Modern Calendar

In the first three parts, we've examined the types and components of calendars, and how the various calendar and dating systems developed within their political and historical context.  In our final installment, we focus on the historical events that led to the development and adoption of our modern Gregorian calendar system.  We also summarize the methods and process of converting dates between different calendar systems and add some final thoughts on the Anno Domini system as it relates to the Christian era.

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 Modern Calendar - Julian to Gregorian

In a previous chapter, we noted that the Julian Calendar was the most accurate calendar ever formulated at the time of its introduction (45 BC).  Yet, it’s year was just over eleven minutes too long (an error of approximately one day every 128 years), so by the sixteenth century, the date for the vernal equinox had shifted about ten days backwards with respect to the March 21 calendar date set at the Council of Nicaea in 325 AD.  This small inaccuracy was causing the date for Easter to drift closer to summer.

The search for a solution began during the reign of Pope Paul III (1534-49) when he began consulting with several astronomers, including the Jesuit Christopher Clavius (1537–1612).  Work continued during the aforementioned Council of Trent (called by Paul III), particularly during the time of Pope Pius V (1559-65).  When Gregory XIII was elected pope in 1572, he commissioned a group led by Clavius to examine all of the suggested solutions offered during the previous years.  After a thorough investigation, the commission recommended Clavius’ solution that was built upon the work of Luigo Lilio (~1510-1576), an Italian doctor and astronomer.

Pope Gregory accepted the committee’s recommendation and in 1582, six years after the death of Lilio, he issued a Papal Bull (Inter Gravissimas) introducing the “New Style” calendar to replace the Julian which was dubbed the “Old Style” calendar.  The New Style calendar would later take on the Pope’s name, and so it is known to this day as the “Gregorian Calendar”.

The reforms in the Bull, an official papal document stamped with a lead bulla (seal), included dropping ten days from the existing date (October 4, 1582 was followed by October 15), moving leap day to February 29, and introduced a new table for determining Easter.  The Julian months, days and years were left unchanged except for a minor alteration in the rules for determining a leap year.  In the Julian system, every fourth year (each year number that is divisible by four) was a leap year.  In the Gregorian system, only every fourth centennial (year numbers divisible by 400) would be leap years containing 366 days.  Thus, the upcoming 1600 and 2000 AD would remain leap years, but 1700, 1800, and 1900 would be regular years of 365 days.

This small change of dropping three leap years every four centuries reduced the annual error between the calendar and the actual tropical (solar) year from the approximately eleven minutes under the Julian system to only about 27 seconds, or one day every 3,236 years under the Gregorian method.

Adoption of the Gregorian Calendar

Today, the Gregorian calendar is utilized almost worldwide.  Yet, in determining adoption dates in various countries, we encounter some difficulties.  There is no single authoritative historical work on the subject, and the adoptive records are better structured in some countries than others.  In a number of locations, the church might have adopted the calendar for religious purposes prior to adoption by the state.  In others, even after adoption by the state, a large portion of the population continued to use the Julian simply for its familiarity.  A detailed chronological list could probably be formulated from the various legislative records, but would also have to take the changing geographical boundaries, political regimes, and demographics into consideration.  With that in mind, we attempt a brief overview of the calendar’s adoption.

In general, the Gregorian calendar began circulating primarily through Roman Catholic territories at the outset.  The Protestant states rejected the calendar initially, but gradually came on board over the next couple of centuries.  The Eastern Orthodox also rejected the Gregorian, and eventually adopted a modified Julian calendar in 1923 that brought its dating closer to the Gregorian.  Muslim and other non-Christian nations initially saw no reason to switch to the Christian-based calendar but, as with the Anno Domini dating system, eventually adopted it over the centuries due to the global economic, political and scientific dominance by Christendom.

Regarding particular nations, only Italy, Portugal, Poland and possibly Spain immediately switched, following Gregory’s Bull by skipping from October 4th to 15th, 1582.  They were soon joined by France and Luxemburg.  Belgium and the Roman Catholic parts of Switzerland and the Netherlands switched either when the Pope sent out a reminder later in the year or shortly thereafter.  The Roman Catholic regions of Germany and Austria, along with the Czechs and Slovakians also adopted the New Style calendar within the first few years, followed by the Hungarians later in the decade.

After Prussia implemented the Gregorian calendar in 1610, it would be another 90 years before any additional significant adoptions.  Around the turn of the eighteenth century, we witnessed the first major Protestant acceptances of the calendar (1699-1701 AD), when Denmark (which included Norway) and the Protestant portion of Germany, Switzerland and the Netherlands made the change.  During this same period, Great Britain made her second attempt at adoption, but after much protest and some violence, this effort like the first (~1645 AD) was unsuccessful.  Even in the states that approved the change however, the Protestants would not adopt the Gregorian Easter tables until the 1770s.

Finally in 1752, England made the conversion to the Gregorian calendar.  By this time, the Julian had passed another 128 year cycle, so eleven days had to be dropped instead of ten (September 14th immediately followed September 2nd).  This legislation also affected all the British colonies, so the soon-to-be America and Canada was now Gregorian.  Sweden then made the switch the following year.

Only two major nations adopted the Gregorian calendar in the nineteenth century, Japan in 1873 and Egypt in 1875.  In the twentieth century however, many nations and territories converted as a result of momentous historical events.  Up until this time, most of central and eastern Europe was under Muslim rule.  With the collapse of the Ottoman Empire about 1912, many of these countries switched to the Gregorian calendar.  Most of those who did not were forced to change by invading armies during World War I.  Even some of the Muslim countries (including Turkey herself) also converted.  Greece would become the last European nation to change in 1928.

Two major superpowers would also change to the Gregorian in the aftermath of communist revolutions (by now, 13 days needed to be dropped from the Julian system).  In 1918, Russia switched after the Bolshevik Revolution and China followed three decades later after Mao’s Chinese Revolution in 1949.

All major nations were now solidly under the Gregorian calendar systems, and dated from Dionysius’ Anno Domini dating system.

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 Closing Thoughts

After reading this four-part article, we hope everyone has a better understanding of calendars, including their components, dating systems, and historical developments.  Before we wrap up, we should say a few words about the methods and process of converting dates between different calendar systems.

The conversion process begins by researching historical, political and scientific (primarily astronomical) records for common “anchor” points between the systems.  An anchor point is an event found dated in both calendar systems.  Dates for additional events can then be established relative to the anchor dates; however this is not always as straightforward as it initially appears.  For example, suppose that we’ve established a few common points between calendars “A” and “B”, and we have a resource document that dates a certain event in calendar “A” exactly 20 years after one of these common points.  We can’t automatically assume that it will be dated exactly 20 years later than the corresponding anchor point in calendar “B”.   We would need to consider all variances between the two calendars’ dating systems, such as differing number of days per year, varying rules for leap months or years, additional periodic adjustments made to synchronize with the seasons, etc.

To complicate matters further, the historian often must harmonize between several calendar systems in order to establish the dates of some events.  This should give us an enhanced appreciation for the scholars who have overcome the various challenges in order to convert ancient biblical and historical events into our modern calendar dating system.

We now offer some final thoughts on the Anno Domini calendar dating system that was created in the sixth century AD by Dionysius while publishing new Easter tables for the Roman church.  Rather than utilizing the existing pagan dating, he originated his system on his calculated year of Christ’s birth.  After all, if pagans could create systems originating with the years of the reigns of temporary earthly kings, why not establish a Christian era system associated with the eternal King of Kings and Lords of Lords.

For this reason, many critics who adopted the Common Era alternate naming conventions claim that the Christian era was created or invented by Dionysius.  In actuality, the Christian era was initiated by Jesus over 600 years earlier when he ushered in the Kingdom of God on earth (Mt 12:28, Lk 17:20-21) at his first advent (the Kingdom will be fully consummated upon His return).  Thus, Dionysius didn’t create the Christian era, he merely dated it.

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