Newton's birthday by today's calendar: January 4, 1643

By the Gregorian calendar used today, Isaac Newton was born on 4 January 1643, at Woolsthorpe in Lincolnshire. English records give Christmas Day 1642, because England still kept the Julian calendar, which then ran 10 days behind the Gregorian calendar already used across most of continental Europe.

Both dates name the same day. Ask a Dutch contemporary such as Christiaan Huygens and he would have said 4 January; ask a neighbour in Lincolnshire and the answer was Christmas. Historians now generally convert Julian dates from between 1582 and 1752 into Gregorian ones, which is why biographies such as the University of St Andrews’ MacTutor list the January date.

The ten-day gap was not England’s only quirk. Until 1752 the English legal year began not on 1 January but on 25 March, Lady Day, so dates in January, February and most of March were written under the previous year’s number.

Newton’s tomb shows it. The Latin inscription in Westminster Abbey says he died on 20 March 1726; the Abbey notes that this is Old Style dating and that the year is 1727 in modern terms. Shift the day as well and his death falls on 31 March 1727. Reference works quoting 1726 or 1727 are therefore reading the same stone differently, not disagreeing about when he died.

Britain fixed both problems with the Calendar (New Style) Act 1750. It made 1 January the first day of 1752, and that September it dropped 11 days outright: 2 September 1752 was followed directly by 14 September. By then the gap had grown by one day since Newton’s birth. So the “Christmas baby” and the “4 January” birthday are one event recorded in two systems, and Newton lived his whole life on the older one.