Did you ever want to know exactly what day a certain date corresponded to?
It's a skill that seems to vastly impress people. Here is a formula
I discovered which does the trick. I need paper to do it, but maybe
you can do it in your head.
You will need:
- A starting date, eg., January 1, 2002 was a Tuesday
- To remember
the following menmonic 303 232 332 323 which represents the different lengths
of months in a year. This is the hardest part.
- To be able to do septal(base-7) arithmetic
Steps:
- Given the days of the week are numbered like this: 0=Sun, 1=Mon, 2=Tue, 3=Wed,
4=Thu, 5=Fri, 6=Sat (this is highly intuitive for me, I hope it is for
everyone.)
- Add digits from the mnemonic beginning at the month you began in, up to but not including
the month you want, for example, if you pass from FEB to APR, add 0+3 (303
232 332 323.)
- Subtract your starting day from the day you want, in base-7.
Examples:
ex1.
Given February 6th is Wednesday(3), what is April 17th?
Feb
to Apr add (0+3=3) 303 232 332 323,
3
+ 0 + 3 = 6
date
17th - 6th = 11, 6+11 = 17 = 37, Therefore,
April
17th is Wednesday (too).
ex2.
Given January 1st, 2002 is Tuesday (2), what is August 22nd?
Travel
through JAN FEB MAR - APR MAY JUN - JUL = 303 232 3
2
+ 3+0+3 + 2+3+2 + 3 = 18 or 47
22
- 1 = 21 (note, 21 is 07) so Aug 22 is Thursday(4).
ex3. Given
April 30th is Tuesday(2) what is May 7th?
This
one is somewhat tricky, because you must go backwards, one way or the other.
April30
to May30 is 303 232.. so add 2, 2+2=4
date
7-30 = -23, -21 is 07, so -23 is -2 or 5 (in base7)
4+5=9
or 27, so May 7th is Tuesday also.