Gregorian and Julian, two competitors in the world of time. They are neither epochs nor eras, they are calendars. The Julian calendar is far older but was more widespread, introduced in 45 BC by Julius Caesar as a revision on the old Roman calendar. It was a revolution, greatly simplified and more accurate, each year was now 365 days with the occasional leap years. Unfortunately it wasn’t accurate enough. There was a discrepancy of 11 minutes, which whilst not sounding like much added up to 10 days every 4 centuries. Something quite significant. So a change was instigated and a new calendar procured, not from the ether but from the Vatican.
A group of mathematicians came up with a separate and more convenient calendar which would be more accurate and not lose any time. In 1582 Pope Gregory XIII unveiled it to the world, announcing that during that year the two calendars would crossover perfectly, causing no confusions. The calendar was named the Gregorian calendar in his honour and was released to the world.
There were sceptics. Even with the wide influence of the Vatican, only eight countries switched over, including Spain, France, the Duchy of Lorraine and Savoy among others.
Then other countries were slowly swayed, with the increasing inaccuracy of the Julian calendar getting no better they saw the benefits. However countries do take a bit of time to realise things. Many countries such as Nova Scotia switched over, losing a day or two because of the delay. The British Empire, and even Eastern USA switched over much later, in 1752.
To their collective annoyance the spacing between the calendars was 11 days. When the switch was made the British Empire went to bed on Wednesday 2nd September 1752 and rose on Thursday 14 September 1752, missing the 11 days in between.
Then there was the reaction of the citizens. Many demanded to be paid full wages for the 11 days and when they weren’t there were widespread riots calling for their 11 days back, as they all believed Britain had stolen the days unjustly. Additionally many contracts due to start on the intermittent days were very muddled up and lawyers were scuttling around for weeks, but eventually things calmed down and the Gregorian calendar reined supreme.
Do not worry though, the Julian calendar still has its uses, it is used for plotting the positions of many religious festivals across much of Europe and Russia due to deeper ties and also for the sake of tradition.