You will often see a year written 1740/41, or 1740 (Old Style). We know that people did not figure out time zones until about a hundred years ago, but surely they knew what year it was much earlier, didn’t they?
Well, yes and no. The problem is that the time it takes for the Earth to revolve once (one day), does not divide equally into the time it takes for the Earth to make one circuit around the sun (one year).
A year takes 365 days 5 hours and 49 minutes. By 57 B.C., some bright spark figured this at 365 1/4 days, only 11 minutes off. That was good enough for Julius Caesar, who about 10 years later made it the official year of the Roman Empire. To catch up the 1/4 day, every 4 years there was a leap year. This was the Julian Calendar. It was confirmed in 325 A.D. at the Christian Council of Nicaea. The Council also determined that Easter, the movable Christian feast that determines so many other dates in the Christian Calendar, would be set at the Sunday falling on or after the first full moon which occurs after the vernal equinox. (The vernal equinox is when the Sun is directly above the Equator as the Earth tilts the northern hemisphere towards the Sun for the summer period. We usually consider this to be on March 21, the first day of spring.)
Since close only counts in horseshoes, the Julian Calendar created a problem. That 11 minutes per year was not much over a person’s lifetime, but it amounted to three days every 400 years. By 1582, it amounted to 10 days. Those scholars whose job it was to calculate Easter were having a problem, because the vernal equinox now came on March 11. At this rate, in a few thousand years, Christmas would be in midsummer, and Easter a celebration of rebirth, would become the start of winter.
Acting on expert advice, Pope Gregory XIII directed that all Catholic countries should immediately drop 10 days from the calendar to bring Easter to the proper place. Also three leap days would have to be dropped every 400 years, by not having leap years in century years where the first two numbers are not evenly divisible by 4 (e.g. 1700, 1800, 1900). This is actually still a few seconds off, but quite accurate.
While refixing the year, Pope Gregory also moved New Year’s Day. Until 1582, it was celebrated on Lady Day, March 25. It was moved to January 1, because recording dates was awkward. Under the Julian Calendar, March 17 was last year, March 25 was this year, but March was the first month of the year. (Remember the old Roman names for the seventh to tenth months, September, October, November, and December.)
At the end of the 1500’s, religion and religious differences were major issues. If your opponents did something, you of course did the opposite. If the Catholics of Europe wanted a new calendar, well the Protestants of England would keep the old one, no matter how much better the Gregorian Calendar worked. Pig-headedness for religious and political reasons has been with us for a long time.
By 1752, the British and their colonies, including America and the West Indies, were now 11 days off in terms of matching dates and seasons. At last, they too adopted the Gregorian Calendar. The day after September 2,1752 was officially September 14,1752, and the new year started January 1, 1753.
When we genealogists see a date such as 3 February 1720, this is actually a Julian date and what we Gregorians would call 3 February 1721. To make sure that we record this correctly, it is often written 3 February 1720/21 or 3 February 1720 (Old Style). Be careful of early Quaker records. They used to refer to 3 February 1720/21 as the 3rd day of 12th month 1720. March was the first Julian month of 1721, even though most of it up to March 24 was in 1720.
And don’t be surprised if your 6 x great grandmother had a baby on 25 March 1715 and another on 20 March 1715. These children are, by Julian reckoning, almost a year apart, not reluctant twins.
The vernal equinox is on approximately March 21 every year now, and Easter, though its dates wander, cannot be earlier than March 22 or later than April 25. If you want Christmas in summer, you will have to go to Australia.