Modern Gregorian Calendar Introduction
The modern calendar in use throughout most of the world today is the Gregorian calendar, named after the Pope who introduced it in 1585. It was not adopted in the United States until an act of the English Parliament made it law throughout the English Empire in 1752. Since that time most other countries have adopted the Gregorian calendar.
Before the Gregorian calendar, the Western world used the Julian calendar. This calendar varied from the modern Gregorian in the leap year rules. Gregory added the 100 and 400 year rules. The Julian calendar had a leap year every four years, without regard to 100s and 400s. Gregory’s calendar omits the leap day every 100 years unless that year is evenly divisible by 400. The year 2000 is the first time in the English speaking world that there has been a leap day on a century mark since the adoption of this calendar.
The following is the shape of the modern Gregorian calendar.
History of this calendar
Gregory’s calendar differs from the previous Julian calendar by only 3 days every 400 years. This seems like a very minor change, and should have been, but in addition to different leap year rules, the Pope changed two other things. First, he tried to correct for drift of the calendar from observed Solar phenomena. Rather than just state that in the future the leap day rules were to be different, he corrected for what he considered to be "extra" leap days that had been observed since the year 300. This meant that several days had to be removed from the calendar when the transition was made.
The British decree that changed calendars said that September 2, 1752, should be followed by September 14, 1752, a loss of 11 days, and that the year 1752 would end December 31 instead of the traditional March 24, a loss to the year of nearly 3 months.
This sort of sudden correction was a different strategy than Augustus had followed in 8 BC. Augustus had decreed that leap days for the next 12 years were to be skipped. This made the transition easier in ancient times. Though Gregory could have chosen this sort of transition in 1585, he chose not to. By the time the British changed the calendar other countries had already changed to the Gregorian calendar. It was impractical to do anything other than dramatically switch as the British did in 1752.
The Pope’s corrections
Interestingly, the Pope only adjusted for extra leap days as observed since the year 300 when Christianity became legal and the church at Rome began to flourish. A more appealing adjustment to Protestant ears would have been an adjustment back to the birth of Jesus. What the Pope had fostered on the rest of the world was calendar centered on Rome, not on Jesus. This was an interesting "trick" and was probably related to the Protestant reformation going on in Europe at the time.
Gregory’s other change was more troubling to countries like England. Christian countries had placed new years at March 25 since the time of Christ. Gregory was proscribing a change back to January 1. January 1 had been new years day when the Roman world was pagan. The Christians of the Roman world had placed new years day on March 25 at the same time they shifted the Sabbath to Sunday. As we explore in Jesus’ Life section, these were both in celebration of Jesus’ resurrection on Sunday, March 25, 31 AD.
Shifting to Gregory’s calendar meant that new years day no longer celebrated Jesus’ resurrection, but it reintroduced new years as a pagan holiday. Sensitive Protestants of the day probably realized that instead of being related to Easter, and the Resurrection of Jesus, new years would become something like what we see in places like Times Square each year.
Protestants probably also realized that Solar phenomena have no particular bearing on standard Christian practice. As most of the other calendars in the world attest, the calendar is usually adjusted to the god of the nation. Pope Gregory had essentially demonstrated that his god was the Sun in the sky, not the Son of God.
These issues probably explain why it took a country like England more than 150 years before adopting this new and pagan calendar. Other countries took even longer. Russia would not do so until the Communist revolution.
Russia still uses the Julian calendar for religious observances. Christmas in Russia falls on December 25 each Julian year. That date lands on January 7 Gregorian when Russians observes the holiday.
When England adopted the new calendar, parliament adopted it completely. New years day was adjusted from March 25 to January 1. This change in calendar introduces reporting errors, where February 1 of any given historical year could fall on one of two years, depending on how the date is reported.
The principle of day-for-a-year end-times prophecy is based in part on the compression of the historical calendar. One ancient year becomes one modern day. Under that compression rule, Enoch who lived 365 biblical years has a compressed life span of 365 days, a modern solar year. That solar year also aligns exactly with 1985 Old Style Julian providing the registration between the Bible’s calendar and all modern calendars.
Note that this suggests God’s "choice" between these two calendar systems is of the Julian Calendar that points at Jesus instead of the pagan Gregorian calendar used across the world today.