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How to be a Human Calendar

Have you ever heard of people who can calculate the day-of-week for any date in their head? How do they do it? Here we explain how anyone can be a human calendar! The basic concept is to add together 3 one-digit numbers, one for year, one for month, and one for date. Then divide the total by seven, the remainder giving a number 0 through 6 that represents Sunday through Saturday. Throw in some shortcuts, and flashcards to memorize the codes, and now anyone can look like a genius!

To start out simple, here are the directions for any date in the 1900's:

Instructions
(for 1900's)
Example:
November 12, 1970
Step 1: Determine the code for the year:

Code # 1 6 4 2 0 5 3 Year: 00 04 08 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88 92 96
Year = 70

(19)68 is 2, so 69 is 3 and 70 is 4!

Total = 4

Flashcards
Step 2: Determine month code:

 Jan = 6 Feb = 2 Mar = 2 Apr = 5 May = 0 Jun = 3 Jul = 5 Aug = 1 Sep = 4 Oct = 6 Nov = 2 Dec = 4
Month = November

November = 2

Total = 6 (4 + 2)

Flashcards
Step 3: Determine date code:

Code # 0 1 2 3 4 5 6 Date: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Date = 12

12 = 5

Total = 11 (4 + 2 + 5)

Shortcut #1: Subtract 7, 14, 21 or 28, from the date to find code, dates less than 7 are already equal to the code.
Example:
Nov 12: 12 - 7 = 5.
Example:
Nov 26: 26 - 21 = 5

Shortcut #2: Simply add the date to the total. Example: Nov 12: Code=12, Total = 18 (4 + 2 + 12).

Flashcards
Step 4: Determine day-of-week from total

Divide the total from Step 3 by seven, the remainder giving the day-of-week:

Remainder: 0 1 2 3 4 5 6 Day-of-week: Sun Mon Tue Wed Thu Fri Sat
Total = 11 (Year=4, Month=2, Date=5)

11 divided by 7 is:
1 remainder 4

4 = Thursday

Shortcut: Instead of dividing, just subtract 7, 14, 21, 28, or 35, etc. (multiples of 7) until you have a number 0 to 6. For example, 11 - 7 = 4. If the total was 30, then 30 - 28 = 2 = Tuesday.

Flashcards
More Flashcards
Step 5: Don't forget leap-year!

Subtract 1 at any point in the process only if the year is a leap year, and the month is January or February!
1970 is not a leap year.

Shortcut #1: The year is a leap year if it is found in the table in step 1. (68 and 72 are leap years, but 70 is not)

Shortcut #2: A year is a leap year if it is evenly divisible by 4.

Flashcards

There you have it! Actually it may take alot of practice before you become proficient, but don't give up! Almost anyone can be a human calendar! Click here for the flashcards to practice the whole process for the 1900's.

Okay, so you have mastered calculating the day-of-week for any date in your head, but only for the 1900's? Click here to show the process when the date is anywhere between 1800 and 2199.

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