10 April, 2008

Perpetual calendar in the head

Today is Thrusday, April 10th, which day of the week is September 3rd? It is actually easy to calculate these things, without the computer's help:

As today is Thursday, 17th, 24th and 31st are also Thursdays. April has only 30 days, so 31st of April is actually May 1st (and it is a Thursday).

Continue with Thursdays: May 1st, 8th, 15th, 22nd, 29th, 36th. May 36th is (36 - 31) = 5th of June.

June 5th, 12th, 19th, 26th, 33rd [-30] = July 3rd, 10th, 17th, 24th, 31st, July 38th = August 7th, 14th, 21st, 28th, 35th, which is September 4th - those were all Thursdays, so September 3rd is a Wednesday.

To go faster we could add several weeks at a time - like 3 or 4 to not overshoot a month completely. 3 weeks (21 days) probably is easier to add, arithmetics-wise, than 28 days.

Let's check October 17th: September 3rd is Wednesday, so September 24th is, too and so is September 45th, which is October 15th. Therefore, October 17th is a Friday.


My Left Nut said...

An alternate method is to flip 6 months ahead on your calendar and just read it off.

Of course, if I didn't have my computer, laptop, pda, GPS or cell phone I guess that your method would work. Or, maybe I'm just too connected to technology.

grundes said...

Oh, it is certainly not a replacement for an electronic calendar, it is for those rare moments when I don't have one on/near me - like when I'm in the bed, almost ready to fall asleep but suddenly have an urge to know which day of the week a friend's birthday will be on.