Strangely enough, there were two calendars in use across the globe during the decade of the Regency. Though most of the world had adopted the Gregorian calendar, there were still some countries, Russia included, which had refused to give up the Julian calendar. Therefore, 1 January 1815 in London was 20 December 1814 in Moscow, as there was a difference of twelve days between the two calendars during the years of the Regency. The lack of a single calendar was to have disastrous results for the allies in at least one battle of the Napoleonic wars.
The Julian calendar, the Gregorian calendar, how they differed, when they were adopted, and how Napoleon turned this discrepancy to his advantage …
The Julian calendar came first. It was named for the man who introduced it, Julius Caesar. He did so to reform the Roman calendar, which had become so complex by Caesar’s reign that many citizens of Rome outside the capital often did not know the correct date. The old Roman calendar often had to be adjusted by the addition of several weeks, whereas the Julian calendar was aligned with the movements of the sun and required no human intervention. It was at this time that the months of the year received the names we still use today.
This new calendar of 365 days went into effect in the year 45 BC and for many years it worked very well. But by the Middle Ages the Julian calendar began to slip out of alignment with the sun. By this time, the calendar was administered by Catholic Church officials, who miscalculated and added a leap day once every three years, instead of every four years. By the 1580s all these extra leap days had caused the calendar to move out of phase with the equinoxes and solstices. That had the result of disengaging the calculation of Easter from the Spring equinox.
Easter, the most important and sacred holiday of the Catholic calendar, was mandated by the First Council of Nicaea to be celebrated on the first Sunday after the vernal or Spring equinox. But by the Julian calendar, the equinox was now calculated at different dates in different parts of Christendom. This was not acceptable to the Vatican, and after much discussion among the church leaders and the most learned astronomers and mathematicians of the time, a calendar reform was inaugurated. Pope Gregory XIII announced the upcoming calendar change on 24 February 1582, by the papal bull Inter gravissimas. Ten days were to be dropped into the new calendar, to correct its alignment with the motion of the sun. Leap days would then be added once every four years. The new calendar was to be named for Pope Gregory.
The last day of the Julian calendar was Thursday, 4 October 1582 and this was followed by the first day of the Gregorian calendar, Friday, 15 October 1582. However, at that time it was only adopted by the Papal States and a handful of other countries loyal to the Catholic church. It took centuries for the Gregorian calendar to be adopted across Europe. The non-Catholic countries essentially ignored the new calendar and kept on with the Julian calendar. Over the next two centuries, more and more countries did adopt the Gregorian calendar, but as you might imagine, there was a great deal of confusion across Europe, with different countries using different calendars.
During the reign of George II, the British Empire, including the American colonies, finally adopted the Gregorian calendar in 1752. By this time, a correction of eleven days was necessary. Therefore, Wednesday, 2 September 1752 was followed by Thursday, 14 September 1752. There is an apocryphal story that the English people rioted, demanding the return of their eleven days. However, there is nothing to substantiate this ever actually happened and most historians give it no credence.
Over the course of the next several decades, other countries followed Britain in adopting the Gregorian calendar. By 1811, when the Prince of Wales became Regent, Greece, Russia, Albania, Bulgaria, Romania, Yugoslavia and the Ottoman Empire were among the countries still using the Julian calendar. But only Russia and the Ottoman Empire were allies in the coalition of other European powers against Napoleon.
In October of 1805, Napoleon was harrying the Austrian army commanded by General Mack von Leiberich near Ulm in Württemberg. General Mack expected to be reinforced by the Russian army, but the Russians were still marching through Poland. They were late joining up with Mack’s army because they were using the Julian calendar, while the Austrians were using the Gregorian calendar. Napoleon was aware of this, and took advantage of the delay to capture the entire Austrian army at the Battle of Ulm. Military historians are divided on whether or not the use of the two different calendars was the cause of the Austrian defeat. Some consider it an apocryphal legend, but others believe this was the cause of the Russian delay in coming to Mack’s aid. The Russians were out of the Napoleonic Wars after the Treaty of Tilsit in 1807, until Napoleon invaded Moscow in 1812. When they began fighting with the Allied Sixth Coalition, there were occasional delays on the part of the Russian army, unless the commanders in charge took the time to "synchronize" their calendars.
If not for Napoleon, there would have been three calendars in use around the globe during the Regency. The French Republican calendar was imposed upon France in 1793. It was intended to sever the ties of the French people from the Church of Rome. The week was ten days long, each day was named by a number and there was no Sunday. But this unique calendar constantly caused problems, even after twelve years of use. Napoleon abolished the Republican calendar and on 1 January 1806, the French returned to the use of the Gregorian calendar. The French Republican calendar was used again, for just eighteen days in 1871.
Princess Lieven, the wife of the Russian ambassador to England during most of the Regency, would have had to keep track of the difference between dates in England and those of her homeland. As would "Hart," the sixth Duke of Devonshire and son of Georgiana. He was a friend of Tsar Nicholas I of Russia and traveled there on various diplomatic missions at the behest of George IV.
Lord Byron would have had to deal with the difference in calendars on his various visits to Greece, which also continued to use the Julian calendar throughout the Regency. Lord Elgin, of Elgin Marbles fame, served as the British ambassador to the Ottoman Empire from 1799 to 1803. He, too, would have had to keep track of the differences in the calendar in the country in which he served and that of his homeland.
Remarkably, the Julian calendar continued in use for more than a hundred years after the Regency. The Julian calendar was not abandoned in Russia until 1918. Greece did not adopt the Gregorian calendar in 1923, and Turkey, the erstwhile Ottoman Empire, finally adopted it in 1926. We in the modern age do have to keep track of the difference in time zones if we have friends and family who live in different countries across the globe. But these days, we all use one calendar, so we have no need to keep track of what day it might be in another country. During the Regency, there were no time zones, those were established by the British railway companies in 1847. But if someone in England had family or friends in a country still using the Julian calendar, they would have to subtract twelve days from the date in England to know the date in that far-off place.