Today there is general agreement throughout the world that for commercial and international political purposes everybody uses the Gregorian calendar, first introduced into the Catholic countries of Europe in 1582. However Europeans should never forget that for other purposes other cultures have their own calendars often wildly at odds with the Gregorian one. Tomorrow is for example the Persian New Year’s a festival, which marks the first day of spring. The Persian calendar is not only used in Iran but in many other countries that were historically under Persian influence. Tomorrow also marks the first day of the year 1394 for Persians. Earlier all cultures used their own calendars a bewildering array of lunar calendars, lunar-solar calendars and pure solar calendars making life very difficult for both astronomers and historians. Trying to find out what a given date in an original document is, or better would have been, on our ‘universal’ Gregorian calendar is often a complex and tortuous problem. Astronomers whose observations of the heavens need to span long periods of time solved the problem for themselves by introducing a standard calendrical scale into which they then converted all historical astronomical data from diverse cultures. Throughout late antiquity, the Islamic Empire and well into the European Early Modern Period astronomers used the Egyptian solar calendar for this purpose. You can still find astronomical dates given according to this system in Copernicus’ De revolutionibus. In modern times they introduced the Julian day count for this purpose.

Within Europe the most famous calendrical confusion occurred in the early centuries following the introduction in Catholic countries of the Gregorian calendar. Exactly because it was Catholic most Protestant states refused, at first, to introduce it, meaning that Europe was running on two different time scales making life difficult for anybody having to do outside of their own national borders, in particular for traders. This problem was particularly acute in The Holy Roman Empire of German States that patchwork of small, medium and large states, principalities and independent cities that occupied most of middle Europe. Neighbouring states were often of conflicting religious affiliation meaning that people living in the border regions only needed to go a couple of kilometres down the road to go ten days backwards or forwards in time. The only people who were happy with this system were the calendar makers who could sell two sets of calendars Gregorian, so-called new style or ‘ns’ and Julian, so-called old-style or ‘os’. Some enterprising printer publishers even printed both calendars in one pamphlet, for a higher price of course.

Within Germany the problem was finally solved at the end of the seventeenth century, largely due to the efforts of Erhard Weigel who campaigned tirelessly to get the Protestant states to adopt the Gregorian calendar, which they finally did on 1 January 1700. England as usual had to go its own way.

Although John Dee, the court advisor on all things mathematical, recommended the adoption of the Gregorian calendar in the sixteenth century the Anglican Bishops blocked its adoption because it came from the Pope and the Anglican Church couldn’t be seen cowing down to the Vatican. Even when the Protestant German states finally accepted that adopting the Gregorian calendar was more rational than any religious prejudices the English still remained obdurate, not prepared to have anything to do with Catholicism. England final came into line in 1752. So what about Isaac Newton?

Many Internet sources are saying that Isaac Newton died on 20 March almost none of them say whether this is new-style or old-style. Most of the sources give 1727 as the year of death a few 1726. Most sources give Newton’s life span as 1642–1727, others 1642–1726 and yet others 1643–1727, what is going on here?

Isaac Newton was born 25 December 1642 according to the Julian calendar that is old-style. If converted to the Georgian calendar, we have to add ten days, and so his date of birth was 4 January 1643 new-style. Things become slightly more complicated with his date of death. Newton died 20 March 1726 according to the Julian calendar that is old-style. Converting to the Georgian calendar we now have to add eleven days because the Julian calendar has slipped another day behind the Gregorian one so his date of death is 31 March 1727 new-style. Wait a minute we just jumped a year what happened here? When Julius Caesar introduced the solar calendar in Rome he moved the New Year from the traditional Roman spring equinox, 25 March, to the first of January. During the Middle Ages the Church moved the New Year back to 25 March. With the adoption of the Gregorian calendar New Year’s Day moved back to 1 January. However England still retaining the medieval version of the Julian calendar kept 25 March as New Year’s Day. Thus at the time of Newton’s death 1727 started on 25 March in England meaning that Newton died 20 March 1726 (os).

Just to summarise if you wish to correctly quote Newton’s dates of birth and death then they are 25 December 1642 – 20 March 1726 (os) or 4 January 1643 – 31 March 1727 (ns).