How to calculate the day of the week when an adult was born

Dedicated F.S.

To calculate like a Rain-Man

-         the day of the week for an adult’s birthday

-         the day they were born

-         the day they will turning 80

-         what day birthday will be this year

-         and what it would have been 100 years

is a very simple calendar feat.

Since in 2013 an adult was born in 19xx you must add only THREE numbers to get the week day:

Month key number, day and year key number. 

We need to convert weekday, month and year into key-numbers and subtract multiples of 7.

Week day key numbers:

For Sunday to Saturday: 0 … 6

Month key numbers:

For January to December: 033 614 625 035

In leap years, January and February reduced by one to 6 and 2!

Day:

Simply use the days from 1 to 31.

Year key numbers (you only need the last 2 digits of the year):

For the years 00 – 99 the year codes do change in a pattern that repeats every 28 years. So you can shorten the year by multiples of 28: 

00 – 27 are the same years as 28 – 55 and so on.

Year:                   
00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83

84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99

Year code: 
  0   1   2   3   5   6   0   1   3   4   5   6   1   2   3   4   6   0   1   2   4   5   6   0   2   3   4   5

For years in 20xx subtract 1. 

One should learn this year codes by heart.

Examples:

I.           June, 27^th 1978

Weekday:

June code is 4; year 78 minus 2*28 is 22, code is 6:

4 (June) + 27 +  6 (year 78) = 37 shorten by multiples of 7 (5*7=35) gives 2

Tuesday.

Turning 80:

Work out which day is always 2 days later:

Tuesday and 2 days later is a Thursday.

This year (year 13 in 20xx; code is 2-1=1)

June, 27^th 2013:

4 (June) + 27 +  1 (year 13 in 20xx) = 32 shorten by multiples of 7 (4*7=28) gives 4

Thursday.

100 years later:

Work out which day is always 1 day before:

Tuesday and 1 days before is a Monday.

II.         January, 15^th 1996

Weekday:

January code is 0 but 1996 is a leap year so subtract 1 ==> 6; year 96 minus 3’28 is 12, code is 1:

6 (Jan in a LY) + 15 + 1 (year 96) = 22, shorten by multiples of 7 (3*7=21) gives 1

Monday.

Turning 80:

Work out which day is always 2 days later:

Monday and 2 days later is a Wednesday.

This year (year 13 in 20xx; code is 2-1=1)

January, 15^th 2013:

0 (Jan) + 15 +  1 (year 13 in 20xx) = 16 shorten by multiples of 7 (2*7=14) gives 2

Tuesday.

100 years later:

Work out which day is always 1 day before:

Monday and 1 days before is a Sunday.