[PBS Bumper] Thanks to Brilliant.org for supporting PBS Digital Studios. [OPEN] December 26, 1937… was a Sunday. It was also the birthday of a mathematician

named John H. Conway. Conway has spent most of his life obsessed

with games, from backgammon to his famous Game of Life… a machine that follows simple

rules, but can evolve incredible complexity. And he can also tell you the day of the week

for any date, past or future, in a couple seconds. Conway is smart, but he hasn’t memorized

1000s of years of calendars. He figured out how to answer this complex

question with a simple set of rules. He invented… an algorithm. And after watching this video–and with a

little bit of practice–you’ll be able to do it too… all with just one hand. Gimme a date! Here’s how it works: First let’s give

every day of the week a number. Here’s an easy way to remember ‘em: NONEday,

ONEday, TWOsday, THREEsday, FOURsday, FIVEday, SIXturday. We’ll use this in a sec. Next, Conway realized in any given year, specific

dates always share the same day of the week. April 4, always the same weekday as October

10. We can make another easy list: For even numbered

months, it’s the same date as the month, unless it’s February, when it’s the last

day of the month. The odd months are a little less obvious,

but just remember “I’m working 9 to 5 at the 7-11” and you’re almost there. All that’s left is Pi Day, and January,

which is the 3rd, unless it’s the 4th in a leap year. And now we’ve got our list: a set of dates

that are the same day, one every month. These are what Conway called Doomsdays. Scary name, but relatively easy to memorize. And since weekdays repeat by 7s, we have a

reference for any day of the year. 2018’s Doomsdays are all Wednesdays. Most years, the weekday for a date moves forward

one day. So in 2019 Doomsdays will be Thursdays. But 2020’s a leap year, and weekdays jump

by two. This is getting confusing. We need to find our Doomsday a different way. Start with the first year of the century. 2000’s Doomsday was Tuesday. Let’s hold that number (2) here, on our

index finger. To find the Doomsday for any other year in

a century was Conway’s stroke of genius. 2057. First, figure out how many times 12 goes into

the year, 4, and hold that number here. Put the remainder, 9, here. Finally, how many times does 4 go into the

remainder? 2. Put it on your pinky. Add these four numbers. If the number’s more than 6, divide 7 into

it as many times as you can, and what’s left over? Seven and Seven and… three. Wednesday’s the Doomsday for that year. For a different century, we just need to change

the index number, or century day. 1900? Wednesday. 1800, Friday. And 2100’s Doomsday will be Sunday. Since our calendar, the Gregorian calendar,

repeats every 400 years, we can figure out any other century, just by remembering these

. Now we have all we need to figure out the

Doomsday for any year, and once we know that, we can figure out any other day that year. Let’s try. EXAMPLES:

What day was the Declaration of Independence signed? Just like 2100, the century code is 0. 76 is 6 twelves, remainder 4, and there’s

1 four in the remainder. 0 + 6 + 4 + 1 is 7 and 4. The Doomsday for 1776 was a Thursday, and

since 7/11 is a Doomsday, so is 7/4. The first moon landing? The century code is 3. ‘69 is 5 twelves, remainder 9, which is

2 fours. July 20 is 2 days after a Doomsday, put it

on your thumb, so 2 + 3 + 5 + 9 + 2 is… 7 and 7 and 7 and 0. It was a NONEday. Kate McKinnon’s birthday’s January 6,

1984. 3, 7, no remainder, and since it’s a leap

year**, 2 days after the Doomsday. If you ever meet her, you can tell her she

was born on a Friday. Congrats, you just performed an algorithm. Algorithms are a set of instructions for solving

a problem step by step. We know them best from computers. But we run algorithms too, all the time. In fact, the word “algorithm” comes from

a Persian mathematician who wrote a book for doing math by hand… the same book that gave

us “algebra”. We’re interested in algorithms getting more

than just the right answer. We also want algorithms to do things efficiently. Lewis Carroll, the author of Alice in Wonderland,

and a mathematician, developed his own algorithm for solving the day of the week. It was printed in Scientific American on June

18, 1887, a Saturday. But Conway’s is quicker. These days, this kind of information is only

a few clicks away, but it shows us how the right algorithms can extract answers from

even complex data, and how, with a little bit of practice, you can be as smart as a

computer. At least for one day. Stay curious. If you’re sitting there thinking “there’s

no way I can do that”, trust me, if you can do arithmetic, you can. I’ve been practicing this method for a few

days, and now I can calculate any date in a handful of seconds. And I’m usually right. If you’d like to practice, we’ve made

this cheatsheet for you… and there’s a non-US version too, because I know we write

dates funny. I hope I’ve sparked your curiosity and showcased

the beauty and fun of mathematics. When I was preparing for this, I couldn’t

simply memorize this formula. I had to critically reason it through to understand

how and why all the various parts fit in, so I could explain it clearly. If you’d like to sharpen your brain and

develop the tools for mathematical thinking, then Brilliant could be the place to go for

that. You don’t just watch stuff, you actively

do stuff. I enjoyed their Joy of Problem Solving course

that presents fun puzzles cut up into easy to digest bits, which can help you think critically

and understand the how and why. It could help you sharpen your thinking up

so much that you’ll be able to cut through any problem put in your way like a… math

Samurai. You can go to brilliant dot org slash be smart

and sign up for free. As a bonus for future human computers, the

first 200 people that click on the link will get 20% off the annual Premium subscription.

Sure, you could just Google it… but where's the fun in that?!

I promise, if you can do arithmetic, you can learn this trick. Let me know how impressed your friends are!

how to know which year is leap year?

Help me out, please

23-1-1967

If nobody kept the time for about a hundred years for some unknown reason, how would we know what day/month/year it is?

There are other tricks to learn…

Okay I get everything except for what you do if the last two digits of the year is less than 12. Does anyone know?

i have seen this trick in Winx lol many years ago

I'm just gonna say now the American way of writing dates makes more sense than the other bullshit. Ascending makes things harder, it's better to go from broad to narrower, leaving yy/mm/DD. However because everyone knows what year it is, the year being at front is unnecessary leaving mm/DD/yy. Just because you aren't used to something doesn't mean it's better. I'm not used to the metric system of measurement, but I can acknowledge it is better than imperial

See…the strange thing here is that Europeans and others can bash Americans for having their dates out of order YET they still use the same generally bassackwards calendar we do. Why is that exactly? To me, it would be easier to have 5 day weeks, 73-week years as those are the only prime factors for 365. To me, if we did that, we'd NEVER have a problem on the week side of things. If we started EVERY YEAR on the same day, we wouldn't have such complicated mind games like this.

it's sooo long…. just look it up on google hahaha

I set it my goal to learn this method. I’ve always told myself I’m too stupid for maths but now I want to prove myself wrong. I’m gonna update if I made it

I gave a thumbs down because he did not explain this well at all:

1) the first example happened to have a 4 as the reminder of 12 and I thought that was what you used for the second reminder, he didn't explain it's always a 4 and this example just happened to be the same number

2) He didn't explain that the doomsday is not necessarily the exact dates each month, but the day off the week its self so I got confused when he did the example with 7/20/1969 and he said that was "2 days after the doomsday" because I kept thinking it was 9 days after it

AAAAAAAAHHHHHHHHHHHHHHHHHHHHHH

NOPE! Nope… nope… nope… nope… nope…

I didn't live long enough to reach the computer age to go back to doing maths in my head.

explain 7/20/1969………9/5=1…but wrote 2. please explain this

Everything works for me except for some leap year January and February's for some reason.

Ex. January 18 1048

5 (cc) + 4 (dividing 12s) + 0 (remainder of dividing 12s) + 0 (dividing 4s into remainder) = 9-(7*1)=2

Therefore doomsday should be Tuesday, January 4th (leap year, 1048 is divisible by 4)

This would put the 18th as a Tuesday as 4+(7*2=14)=18. This is untrue, however, as the calendar says this date should actually be a Monday. What am I doing wrong?

It would have bern a good tutorial if you hadn’t been in such a hurry, also insread of the finger nonsense it would have been easier for me if the procedure had been written on a board, step by step… but I forget you don’t think that older people might want to understand as well… I’m 75 and I was left in the dust…I hope that makes you happy to know that…

I tried it at least 4 times, and I was wrong every time…

I figured it out YAY

?

How to find century code?

I understood everything until when he started explaining it.

I mastered this algorithm, because of you guys! Thanks for teaching.

I've been using the other method mentioned for a while now. This is so much faster, I'll definitely be switching to this one now

At 4:19 century code should be 3, not 0

Dude has that special public school teacher talent for confusing those who want to learn. (I reviewed the video and deciphered the information after an hour, though).

You just single handedly confused everybody who was playing along by using the inverted date format

over my headThank you Kanye, very cool

How to get century code?

I'm pausing the video every now and then ??? anyway thanksss ;))

Awesome trick! But how can we do it with years that are less than 12? And what is the deal witg leap years? Is it just me that cant make it work when its a leap year?

Ain't nobody got time for that.

My birthday is on a doomsday……

My friends birthday is 6/6/6

Pi day is my birthday!

I dont think I'm smart I feel like I'm dumb 1 like =smart

Love this channel!

I like these things, have had a nerdish interest in this ever since I saw an old formula by Gauss, when I was about 10 years old. 🙂

There are many ways to do this, and personally I find this doomsday thing a bit awkward to keep in my head, I remember devising a different system but can't for the life of me find it now, and have long since forgotten.

What I did do at one point, that others have had use for is more of a simple step by step table based version, where one just follows along, without having to memorise anything, can be found at https://app.box.com/v/weekday should anyone be interested. 😀

Can you tell me when Soloman Grundy was born though?

Isn’t it a Friday at 4:28?

How to know the century code

plz DD/MM/YYYY or its confusing af

4:47 … As per the fact goes, he learnt algebra from India and then wrote a book!!

1:08 Picard is better than Kirk

I am born on 18th Aug, Friday

My way of solving what day of the week it what is the: '' Moving Down The Week Method''

I find this video incredibly insensitive to those of us without hands.

I didn't understand anything

I dont understand

I don't get the century code

I wrote a day-of-the-week challenger.

It asks you the weekday of dates, you choose one and it tells you if you're right:

https://ndsvw.github.io/Which-day-of-the-week/

I didn't understand how to calculate doomsday for century

How did it work for 19th October 1902 I tried but didn't get that a Sunday????

1500 doomsday is saturday(6)

how do I know if it's a leap year or not?

Monday is One day

Tuesday Two day

Wednesday is What? Where? Who? Day

Thursday is Three Day

Friday Four Day

i take too long

why does the guy at 0:57 sound so much like jason from gtlive?

You could also use Zellers rule: f = ?((13*m)-1)+d+(d/4)+(c/4)-(2*c))/7

Tnteresting, even though nuts, as who really cares about thus.

However, sorry nerdy indeed …

I didn’t get the remainder thing

The week starts with Monday, NOT Sunday. Sunday is the end of the week, hence Weekend (Sat and Sun). Monday is when the week starts

4:06 how's july 20th two days after doomsday, ie. july 11th?

I really dont get it, how you know 20 july is 2 days afterdooms day? i read on that link 7/11

sry for asking

How tf do you know if the year is a leap year

I made it to four minutes in. This was a really stupid waste of time. You guys can definitely do better.

This ( https://youtu.be/TKSpLH9CK_c) is way easier. Just subtract 1 from the FINAL ANSWER in case of leap year.

Whoa… this is much simpler than calculating the odd days to find the day of the week. Thanks for sharing this method!

Is it applicable to 14th century?

how to calculate the century code for a particular year

How to calculate century code

Why month-day-year format? Just use football fields.

How do you make the numbers float on your fingers like that? I’m stuck ??

I gave up at 1:56.. nopers

I sitll have no idea

I was doing okay or so I thought initial he said the remainder but dosen’ specify where the 48 came from.

the declaration of independence was signed on the second of august

How about years like 2000-2011 or 1900-1911

Impressive. However I don't see myself using this skill anytime soon

I thought this was going to be an easy trick like using your knuckles to find out which months have less than 31 days

1:16 Thought I was listening to Joey explain how to remember what day of the week it is to Chandler

"ez to memorize"

I ate my dog.

Like if you get the reference.

This is how savants do it

Learned this in about 20 mins ?

…I fan solve a rubix cube in 1minute 13seconds using algorithms, but I got lost in this video… I'm going to go ear some rat poison now.

Tootles

An I don’t need to know this lol

The only thing i cant get my head around is the leap years and if it was a year like 2003 so how would you work it out if you have the date June 27th 2003?

What's a century code?

Someone figure out 7th October, 2012….I'm stuck with that…..

i keep trying but i cant seem to have it work for new years day. i tried using 1/1 1949(saturday), so century code (3) 12 goes into 49 (4) times the remainder is (1) and 4 goes into 1 (0) times 3+4+1+0=8. 1/1 isn't a doomsday and is 6 days after the last doomsday(december 26th 1948) 8+6=14. 14-7-7=0 or sunday. is there something that needs to be used for new years day to find out what day of the week it is/was

When you get lost at the bullshit like having mm/dd instead of dd/mm and when week supposedly starts on sunday??

Guys it's simple, just got it, finally!

If the year is below 12 (exm. 01; 02; 03; 04; 05; 06; 07; 08; 09; 10; 11) you just take it, not divide it by 12, then the magic works 🙂 Enjoy everyone!

How to know which year is the leap year.

This video has me wondering if it's okay to be stupid

what if the last two digits are smaller than 12

Thanks you help me man. I use to do the doomsday the hardware

you explain it too quickly i can't follow it

i need this algorithm for an exercise of my computer class

Just awesome