Prior to 46 BCE, the Roman calendar was being exploited by politicians and others who haphazardly added days or months to it. As a result, calendar dates didn’t match the seasons of the year. Unlike the calendar, which is an artificial creation of humans and subject to political influence, the seasons of the year are based on the time it takes the earth to go around the sun, a fact which remains oblivious to human legislation and whim. To bring Order out of Chaos, Julius Caesar took control of the Roman calendar and decreed that henceforth the calendar would be 365 days long with an extra day to be inserted every four years. The extra day was to be added prior to February 25. This seemed to solve the problem and the calendar now matched the seasons.
BUT WAIT: Caesar’s calendar was fairly accurate, but it assumed that the year is 365 days and 6 hours long. In fact, not subject to human legislation, the year is approximately 365 days 5 hours 48 minutes, and 46 seconds long. Therefore, the calendar of Julius Caesar was 11 minutes and 14 seconds too slow. This added up to be a full day off every 128 years.
Some 1,600 years later, the calendar was noticeably out of sync with the seasons and causing some concerns among Christian religious leaders. Easter, a European pagan holiday which had been claimed by the Christians, was supposed to be based on the date of the vernal equinox (i.e. the first day of Spring). According to the calendar, however, Easter was celebrated too early in March.
In the year 1572, Ugo Boncompagni became Pope Gregory XIII. By this time, the calendar was off by ten days. It was obvious that a serious change needed to take place and Pope Gregory XIII set out to repair the ailing calendar. In consultation with astronomers, a new calendar was created. Like the old calendar, the new Gregorian calendar would continue to be made up of 365 days with an intercalary added every four years. The extra day, however, was moved to after February 28 to make things easier. The solution to keep the calendar in long-term sync with the seasons called for no leap years to be added in years ending in “00” unless those years were divisible by 400. Thus, the years 1700, 1800, 1900, and 2100 would not be a leap years but the years 1600 and 2000 would. This change was so accurate that today, scientists need only add leap seconds every few years to the clock in order to keep the calendar matching the tropical or solar year.
Coming up with a more accurate calendar and getting people to use it are two different problems. Pope Gregory XIII issued a papal bull, “Inter Gravissimus” on February 24, 1582 that established the Gregorian calendar as the new and official calendar of the Catholic world. In order to get the new calendar in tune with the seasons, Pope Gregory XIII designated that October 4, 1582 would be officially followed by October 15, 1582. In addition, the new year would now begin on January 1 rather than on March 25.
Not everybody liked the new calendar. It was adopted that year in Italy, Luxembourg, Portugal, Spain, and France. The Pope was forced to issue a reminder on November 7 to nations that they should change their calendars and many did not heed the call. Unfortunately for the Pope, Protestantism had spread across Europe and there were now many people, including political leaders, who no longer heeded his Papal Bulls.
Roman Catholic Germany, Belgium, and the Netherlands switched by 1584; Hungary changed in 1587; Denmark and Protestant Germany switched by 1704; Great Britain and its colonies changed in 1752; Sweden changed in 1753; Japan changed in 1873 as part of Meiji's Westernization; Egypt changed in 1875; Albania, Bulgaria, Estonia, Latvia, Lithuania, Romania, and Turkey all changed between 1912 and 1917; the Soviet Union changed in 1919; Greece switched to the Gregorian calendar in 1928; and finally, China changed to the Gregorian calendar after their revolution of 1949.
Russia up until the Communist Revolution was under the Russian Orthodox Church which did not accept the Gregorian calendar. By the time of the revolution, the calendar in Russia was about ten days out of sync with the seasons and so the October Revolution was really November in other parts of the world.
The change in the calendar prompted riots in places like Frankfort and London where people were concerned about the loss of days in the lives. Laws had to be passed decreeing that people could not be taxed or paid during the “missing” days. Furthermore, interest could not accrue on these days.
In Great Britain, Parliament debated the change to the Gregorian calendar (called the New Style calendar at this time) in 1645 and again in 1699, but both times the proposed legislation failed to pass. Finally, in 1751, Parliament legislated the change, but needed to add 11 days instead of 10 to get the new calendar in tune with the seasons. At this time, both the United States and Canada did not exist and thus the new legislation applied to all of North America, except for Alaska which was a Russian colony.
Following the change in the British Empire, people would write dates with the designation O.S. (Old Style) or N.S. (New Style) following the day so people examining records could understand whether they were looking at a Julian date or a Gregorian date. George Washington was born on February 11, 1731 (O.S.), but under the new Gregorian calendar his birth date became February 22, 1732 (N.S.). The change in the year of his birth was due to the change of when the change of the new year was acknowledged. Previously, March 25 had marked the new year, but with the new calendar this was changed to January 1. Therefore, since Washington was born between January 1 and March 25, the year of his birth became one year later upon the switch to the Gregorian calendar.