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Doomsday rule: calculate the day for any date (wikipedia.org)
125 points by iamwil on Aug 10, 2014 | hide | past | favorite | 30 comments



Freshman year of uni I memorized some variant of this, and after some practice I got to where I could do the math in 3-5 seconds.

Protip: Some parts of the algorithm can be subject to a CPU/Memory trade off. In the variant presented in the wiki article, steps 1 and 2 can be combined. Memorize the doomsdays for all of the years of people you're likely to want to calculate their date of birth. For me, this meant memorizing the doomsdays between say, 1980 and 1995.

You can also memorize a modulo 7 table for all numbers you're likely to encounter.

You can get fast enough in the amount of time it takes you to say some canned response "Hmm, I think the date X YYth of ZZZZ would fall on..." you can figure out the answer.

-----

Though it turns out this is a significantly less impressive party trick as I initially imagined it'd be.


Here's my formula for D/M/Y where Y=YYYY=CCFS

  (CC=century digits F=First S=Second digit of the last two digits in year)
Note:Only for Gregorian calendar.

Formula= {D +Table(M) -1 IF month=1or2&leap_year +Table(CCmod4) +Table(F) +Table(S +2 IF F is odd) } mod7

Tables are:

  Month {033614625035} or Jan=0 Feb=3 Mar=3 Apr=6 etc.

  Century {6420} or 16xx=6 17xx=4 18xx=2 19xx=0 20xx=6 etc.

  TableFirst memorize {0340145025} or {00,13,24,30,41,54,65,71,82,95}

  TableSecond memorize {012356013456} or  {00,11,22,33,45,56,60,71,83,94,105,116}

  FormulaResultTable:1=Monday...6=Saturday 0=Sunday
Example: 16 August 2014 = (16+2+6+3+0)mod7=6=Saturday

In this method,I find slight advantage over "odd+11 method" because my method does not involve two digit arithmetric (and memorizing running total if you're not calculating FS first),although learning tables take some time.


People aren't impressed by it?


Only a certain kind of people really appreciate it.

Generally, the same kind of people who are impressed by rubik's cube speedsolving (which I also thought would be amazingly cool) are impressed by this. But for a lot of people, they just assume there is some simple trick to it, say "cool" and move on.

And then other people assume you must be some sort of autistic savant.


I found the same for the ability to divide numbers by 17 in my head... it really didn't get me a lot of action.


It's hard to know if the answer you are given is in fact correct unless you knew it beforehand.


Fortunately, I've found that most people know which day of the week they were born on. Once or twice, though, someone was mistaken and upon consulting wolfram alpha, we determined the doomsday algorithm was correct.


Most people are impressed by what a person owns or looks like, not what they are capable of or what value they can add to the world.

It saddens me.

I'd sit with someone who did this and learn how it works!

My party trick clears the room: eye socket farts.


Not sure how the doomsday works, but there's a quick way to calculate the day of week just memorizing single digit codes for months. Might be easier to grok from a small script. This is for current century (2000s) in coffeescript:

calculateDay = (year,month,day) ->

  y = (year-2000)%28
  m = [6,2,2,5,0,3,5,1,4,6,2,4][month]
  if month < 2 and y%4==0
      m-= 1
  y += y/4|0
  days = ['Sun','Mon','Tue','Wed','Thu','Fri','Sat']
  return days[(y+m+day)%7]
--- For years 1600s through current century here's a jsfiddle with a slightly longer method with verification: http://jsfiddle.net/9zp0zkxz/

Edit: no idea how to work spacing on comments here


  > no idea how to work spacing on comments here
* Blank lines create new paragraphs,

  Blank line followed by lines with
  leading spaces create pre-formatted
  text - intended for inserting code.
Under most circumstances, asterisks around text creates italics, the exception is when the asterisk is surrounded by space.

-- https://news.ycombinator.com/formatdoc


ah, thank you


  calculateDay = (year, month, day) ->
    y = (year - 2000) % 28
    m = [6,2,2,5,0,3,5,1,4,6,2,4][month]
    m-- if month < 2 and y % 4 is 0
    y += (y / 4) | 0
    ['Sun','Mon','Tue','Wed','Thu','Fri','Sat'][(y + m + day) % 7]


The 'Big Calendar' industry is going down.


Big Calendar is already on its way down. How many of us even have calendars?


When I wrote my time and date library (luatz: https://github.com/daurnimator/luatz ) I originally wrote an implementation of the Doomsday rule to figure out the day of the week.

Turns out that sakamoto's algorithm (see https://en.wikipedia.org/wiki/Calculating_the_day_of_the_wee... ) was much faster to calculate and made for cleaner code. https://github.com/daurnimator/luatz/blob/master/luatz/timet...


IIRC tzdata just acquired a new version (3) not too long ago and luatz would need to be updated to handle it. You'll only get the new files if they were compiled with a newish zic. This bit me because code I work with is also parsing them :)


Thanks for mentioning this, I had no idea... and documentation on this fact is sparse. It's not even mentioned on https://www.iana.org/time-zones/repository/tz-link.html

Created an issue at https://github.com/daurnimator/luatz/issues/2


Right, the Doomsday Rule is optimized for humans, not computers.


It seems the algorithm would be better optimized for humans if January and February were always considered to be the last two months of the previous year. We're already used to doing this when working out birth years for Horoscopes and the Chinese calendar. Then we wouldn't need to memorize different dates for Doomsday in Jan and Feb, and instead remember Jan 2, Feb 6, etc.


Why are those days called 'doomsdays'? Anybody knows? (The origin articles don't seem to be online.)


Maybe because its inventor is John Conway?

You know, "Conway's Game of Life", "Doomsday rule"... He's not into small names for the things he invented.


I do believe the etymology in this case lies with Old English 'domes dæg' which was thought to be some apocalypse 6,000 years after creation, which was thought to be ca. 5,200BCE.

As for your question why, I cannot reasonably guess. :-)

Somewhat related, you may be interested to read up about the Doomsday Book.


As I explained to a friend recently, I'm not the type of person who can readily do the calc in my head, but rather the type who would enjoy the tedious work of laying out all the possibilities on paper, as I did with the Periodic Calendar: http://periodiccalendar.com/


"Pick a date, any date."

"June 12, 2145."

"A Saturday. In Vancouver, sunrise will be at 5:12am and sunset will be at 9:24pm. There will be a third quarter moon."

"Wow."

http://www.wolframalpha.com/input/?i=June+12%2C+2145



The First Sunday Doomsday Algorithm is the easiest to learn and also fastest, in my opinion. http://firstsundaydoomsday.blogspot.com/


This script will test your skills on logon: https://code.google.com/p/doomsday-sh/


The periodic calendar provides a map to this math: http://periodiccalendar.com/learn/


Thanks, Jeff! The key to the Periodic Calendar is identifying that there are seven types of years, depending on the day of the week that starts it off. At first I thought I needed to make 7 calendars to cover the possibilities, but my aha moment came with the idea of the Gregorian Isotopes, which flip our focus from the day of the month to elements based in days of the week that exist on an array of days of the month.

The core idea is that August 10th is a semi-meaningless count of the days, whereas the real information lies in today being Sunday.


This rule is great for convincing people that you are autistic.




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