## CN. Name That Date

John. H. Conway, one of the young men pictured above, tells us about his fabulous and simple method for rapidly calculating the day of the week.

With just a little practice, you too can Impress your friends (or drive them away) with this stupendous ability!

1. ### strauss said,

May 27, 2007 at 10:49 am

In the epsiode, we talked about how to find the day of the week of any date in the XXth Century, a “Wednesday Century”. The current century is a “Tuesday Century” and the method works the same way, counting from Tuesday.

But we didn’t mention what kind all the other centuries are:

In the Gregorian Calendar, the centuries roll around thusly:

1200 1600 2000 2400 etc are Tuesday centuries.
1300 1700 2100 2500 etc are Sunday centuries.
1400 1800 2200 2600 etc are Friday centuries.
1500 1900 2300 2700 etc are Wednesday centuries.

However, today is Sunday May 14, 2007, in the Julian Calendar, and the Julian centuries work thusly:

000 700 1400 etc are Sunday centuries
100 800 1500 etc are Saturday centuries
200 900 1600 etc are Friday centuries
300 1000 1700 etc are Thursday centuries
400 1100 1800 etc are Wednesday centuries
500 1200 1900 etc are Tuesday Centuries
600 1300 2000 etc are Monday centuries.

In many Catholic countries, the day after Thursday October 4, 1582, in the Julian calendar, was Friday October 15, 1582, in the Gregorian. This change came later in other places; the British Empire switched nearly two centuries later, and Russia only in 1918. Wikipedia has a fascinating discussion of this history.

Curiously, then, all would agree that the English defeated the Spanish Armada on a Monday but was the date August 8, 1588 (as according to the Spanish) or July 29, 1588 (as according to the English)?

2. ### mathphan said,

March 28, 2009 at 3:51 am

There’s an interesting oddity in the changeover from the Julian to the Gregorian calendar. As you may know, different countries decided to switch at different times.

Sweden started to make the change from the Julian calendar and towards the Gregorian calendar in 1700, but it was decided to make the (then 11-day) adjustment gradually, by excluding the leap days (29 February) from each of 11 successive leap years, 1700 to 1740. In the meantime, the Swedish calendar would be out of step with both the Julian calendar and the Gregorian calendar for 40 years; also, the difference would not be constant but would change every 4 years. This strange system clearly had great potential for endless confusion when working out the dates of Swedish events in this 40-year period. To make matters worse, the system was poorly administered and the leap days that should have been excluded from 1704 and 1708 were not excluded. The Swedish calendar (according to the transition plan) should now have been 8 days behind the Gregorian, but was still in fact 10 days behind. King Charles XII recognised that the gradual change to the new system was not working, and he abandoned it.

However, rather than proceeding directly to the Gregorian calendar, it was decided to revert to the Julian calendar. This was achieved by introducing the unique date 30 February in the year 1712, adjusting the discrepancy in the calendars from 10 back to 11 days.

That’s the only time in history there was a February 30th, so if you just happened to be born in Sweden on that day, you would never age! You would never have a birthday for as long as you lived.

Sweden finally adopted the Gregorian calendar in 1753, when Wednesday, 17 February was followed by Thursday, 1 March.