The Doomsday Algorithm - Numberphile

Featuring James Grime.
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[@mzadro7]

That “wowie” made James sound like the most un-surprised surprised person in the world

[@bryantg8749]

He's always the type of person that makes you go "wow" and you agree with him as you say it to yourself.

[@MarkWatney]

I think Ron said it too during the first train trip in Harry Potter and The Philosopher's Stone.

[@kristianemilkjrgaard531]

Or like Mr. Poopy-Butthole from Rick and Morty

[@spacebusdriver]

sounded exactly like wilburgur ngl

[@oscargill423]

"Oh wow what a surprise I wasn't expecting that at all"

[@minigrok]

John Conway, the mathematician who made up this algorithm, used it as his login for his computer at his office at Princeton. The computer would give him 10 random dates in any century and would not let him log in unless he got them all correctly in under 40 seconds. He managed to do all 10 in about 15 seconds.

[@asterism343]

how do you know

[@charlieangkor8649]

that's not very secure

[@theterribleanimator1793]

@@charlieangkor8649 but it is cool.

[@minigrok]

​@@asterism343 Conway showed me, I'm just relaying first-hand info. His main research assistant also did the same. This was around 1995.

[@minigrok]

@@topherthe11th23 Yes, it was 10 dates not 15. I edited it now.

[@Cubeorithms]

This is crazy, after less than an hour of practice I'm getting it right almost every time. Those leap years are tricky though

[@jasmintoromanovic5975]

Nice

[@PlutosMusic]

bro ur really copying and pasting your comment

[@theevauwu7853]

I still can't get the anchor year to work I keep getting it wrong

[@PerspectiveInsightReviews]

Me too, I can't believe it's so easy

[@mahiiii2112]

i have a doubt, how to know how many leap years to add 10:41.. im really confused please help me

[@blakehagins3069]

doomsdays: 3:35 calculate doomsday for arbitrary year: 6:21 day of week to number conversion: 8:13 doomsday century landmarks: 9:25

[@Dr.Cassio_Esteves]

Up

[@davidroddini1512]

He shows how to calculate for dates AFTER 2000. I can do that. Trying to go back I have to actually count the leap years 1996, 1992, 1988, etc or I get the dates wrong. What am I missing? Why is it not working doing subtraction instead of addition?

[@collinvanweelden5242]

@@davidroddini1512 you have to subtract a leap year every time you go back by a multiple of 4 but starting at one. That is because 1999 to 2000 is a leap year so going back you have to go back a leap year also

[@jaysonsvan6092]

I don't get the day of week to number conversion... I understand better with examples😅 can anyone help me out?

[@johndododoe1411]

@@jaysonsvan6092 Monday is 1, Tuesday 2. Sunday is 7th and last, reduce mod 7 to get 0.

[@TomRocksMaths]

YES James is back. Mr Numberphile

[@thatguyalex2835]

James is the reason I watch Numberphile, as well as the fact that this channel has pretty informative math content. :)

[@fariesz6786]

the number of people associated with numberphile who i have a man-crush on is improbably high xD

[@ragnkja]

The very first Numberphile presenter is back!

[@Triantalex]

false.

[@alistairkentucky-david9344]

“He remembered that 0 is a 0”. Well that just confirms that memorising this whole algorithm is above my pay grade.

[@renerpho]

9:00 Note this is for the Gregorian Calendar, so be careful with early dates. For England, the calendar change took place in 1752, so this method only works for dates starting in 1753. For Russia, dates prior to 1918 don't work, for the same reason.

[@davidkim6673]

But all you have to do is just to remember a different century date schemes, and you can convert to a Julian calendar!

[@renerpho]

@@davidkim6673 To some extent, yes. But you'll also have to know which calendar applies, and that's highly dependent on where the event in question took place. Especially on the European continent, the time when territories changed calendars can vary by several centuries between neighbouring towns. There are tables that tell you what date which town made the switch, but try to memorize hundreds, if not thousands of entries...

[@cOmAtOrAn]

@@davidkim6673 The real difficulty is memorizing when every country in the world converted to the Gregorian calendar. And it is something that would have to be memorized, there's no pattern to be picked up on.

[@renerpho]

Of course you could circumvent the difficulty, by asking the person whether their date is Julian or Gregorian.

[@TheOneAndOnlyOuuo]

@@renerpho I live in an environment where coming in contact with the Julian calendar is an every day thing even today. You're free to guess where I live ;)

[@Kranzio-]

I’ve always loved the little Numberphile thumbnail caricatures that manage to be both recognizable and strangely unsettling.

[@vigilantcosmicpenguin8721]

Whoever the artist is is perfect for the channel.

[@somedudeok1451]

Especially when it has the word "Doomsday" written next to it. I was sure this video was gonna be a lot darker than it turned out to be.

[@ChristainGuyOfAction]

I believe the term is "uncanny valley."

[@drenz1523]

And James' caricature is the most uncanny, just starring at me

[@mr.joesterr5359]

The thumbnail for this video is an absolute monstrosity from the darkest trenches of the abyss itself

[@davidfalterman8713]

James coming through with the NUMBERWANG reference at the end killed me 💀

[@cloudstrife1983]

Me too

[@ericwiddison7523]

Now let's rotate the board!

[@YourCrazyOverlord]

Don't forget your Numberhosen

[@thetooginator153]

I am proud to be an American who knows what Numberwang is (and Colosson!).

[@atrusx8232]

The comment I was looking for.

[@Leo-if5tn]

I really apreaciate when this channel presents James, hope he returns more often

[@papafreddy2123]

One of the neatest party tricks I've ever seen, maths being fun as usual

[@somedudeok1451]

Yeah. Had a friend once that would always bust out some intriguing riddles and tricks at parties and I loved that guy. Somehow these tricks are even more impressive when you're drunk. :)

[@user-sg1kc5jx8c]

Hey can you tell me how to subtract dates? For example they say 8th February. How do I get from 28(doomsday) on a non leap year to the day of the week?

[@mblada4514]

​@@user-sg1kc5jx8c if the 28th is a doomsday, the 7th will also be a doomsday(because going back 7 days doesnt change the day of the week), so the 8th will be one day after doomsday

[@pvandewyngaerde]

30 + 31 + 2 = 63. This is a multiple of 7. This explains why the even month days are the same.

[@msolec2000]

Correct. And those 63 are split into 28 and 35 by the odd dates. :)

[@KaiHenningsen]

The really surprising thing (back when I found out, many decades ago) was that 400 years are a whole number of weeks - this is why an eternal calendar works. Fast check: 1 year = 52 weeks + 1 day, so 400 years makes 400 extra days; every leap year is another extra day, so one per 4 years makes 100, less one per 100 makes 4, plus one per 400 makes 1, add all up makes 497. Which is 7*71. So indeed, 400 years are an exact multiple of one week. I seem to recall that you can also verify that Friday the 13th is happening more than average in some respect based on this, but I've long forgotten the details. Oh, and don't forget that this is only true for the Gregorian calendar, not for the Julian, so make sure you don't go farther in the past than whenever the Gregorian calendar was adopted at that place! ETA: various typos

[@chriswebster24]

Yep, and 26 + 17 + 5 = 48, which is a multiple of 8. This explains absolutely nothing, but if you add 21 you get a fun game that couples sometimes play.

[@therealax6]

Bonus points for showing why it works when July and August, which are consecutive, have 31 days each. (Hint: why doesn't it work for the odd months?)

[@maxono1465]

Where does the +2 come from

[@harrypainter7472]

1:15 Most convincing wow ever

[@pvic6959]

when you do well on a test that you thought you failed lol (im long out of school but that feeling stays with me)

[@blue_tetris]

WoW-wOwEeEe

[@sshuggi]

This should be a drop in future videos as a little Numberphile/Bradyverse meme.

[@_rlb]

That needs to be a gif

[@__gavin__]

That's numberwang!

[@mina86]

Let’s rotate the board!

[@mollycollins2125]

Came from Mike Boyds channel

[@somerandomweeb4836]

Finally, the return of James Prime

[@thenasadude6878]

James Prime, leader of the MACSYMA, fighter against the Decepticons

[@NoriMori1992]

@@thenasadude6878 Also known as OCTOMUS PRIME

[@LouisOnAir]

It's quite nice that after a full 400 year cycle of years and leap years (X is a leap year IF [4 | X & NOT 100 | X] OR 400 | X), Doomsday doesn't change, it's always Tuesday on the multiples of 400.

[@arnaldo8681]

it means the number of days in 400 years is a multiple of 7. I found it to be the most surprising thing in the video

[@gabor6259]

This is true only from the adoption of the Gregorian calendar.

[@d.m.collins1501]

And even if the dates WERE Gregorian, the Romans and their descendant nations didn't adopt the Sunday-Saturday week until 321 a.d. when Constantine was like "okay, let's do what Christians do." Before that, the Romans used an A through G date designation. And a couple hundred years earlier, they actually had 8-Day weeks!

[@Scanlaid]

"It's a bit numberwang" 😂 hilarious. It was, but still such a cool trick

[@Rubrickety]

But can he _prove_ it’s Numberwang?

[@GGoAwayy]

Das ist Numberphile!

[@kenthemaster]

That's Wangernumb!

[@EnteiFire4]

If you keep track of the day of the week with a number, here are some great mental shortcuts: - When adding the numbers together, you can pre-remove the extraneous 7 (AKA compute the number modulo 7). So for example, 20 + 37 would become 6 + 2 (because 20 = 2*7 + 6 and 37 = 5*7 + 2). - "High" numbers can be converted to negative numbers. For example, a 6 can be replaced by a -1 and a 5 by a -2. It's not that easy to do 6 + 5 modulo 7 quickly, but -1 + 5 = 4 is easier.

[@therealax6]

...and don't be afraid of keeping the extra sevens. It might be easier to add 10 than to add 3. 20 + 37 is instantly 57, and in that case, it's faster to do the modulo once at the end.

[@virenor]

Yes, I just calculated my first date now and my doomsday happened to be 13, so I spontaneously converted it to negative 1 instead of 6.

[@diegotrabanino8970]

This is a great shortcut! Thank you

[@thecalendarninja]

👍 nice, i thought i was the only one who took advantage of using negative numbers to cancel things out faster

[@JonBastian]

I love this one, particularly back in the days of live meetings, because someone might ask a question like, "What day is Halloween this year?" and without checking or hesitating, I'd just answer It only took a few times before people would stop checking on their phones because they knew I was right. I never really mastered the giving the day for a date in a particular year trick, but since is the first clear and concise explanation of that part that I've ever seen, I'm going to start working on being able to do it. Thanks, professor!

[@KayvanAbbasi]

James Grime, why I originally started watching Numberphile probably 8 years ago. Still, an excited man and exciting to watch. Fun fact: He does not age! Knock on wood! :)

[@ed6213]

@@epsi So THAT"S why he finally came out on October 31!

[@chriswyatt9869]

Mike Boyd bought me here

[@nzwakele]

I want to understand this so bad but i hvnt done math in 12 years

[@MichaelaBennison]

This was an episode of "Would I Lie to You" - Lee Mack had to convince the opposing panel that he could say the day of the week of any date. He was lying though.

[@FCBanton]

Yeees! I immediately thought of him. Turns out it's actually quite possible!

[@stomponpie]

I sort of like the idea that some producer thought there was a tiny chance Lee could come up with something like this on the show and fool everyone.

[@woutervanr]

I was just looking through the old videos and wondering when James Grime was going to turn up again and then this is posted. What a coincidence!

[@TheZenytram]

calling "Tuesday" as "Twosdays' completely broke my Portuguese brain.

[@shirou9790]

that said the method will of course still work if you choose Sunday = 1 rather than Sunday = 0, which would indeed be way easier in Portuguese

[@lhaviland8602]

Just wait for 2/22/22...

[@harmidis]

Right! In Greek and also in Portuguese Tuesday is the 3rd day, so it is called "Τρίτη" or "terça"

[@therealax6]

@@lhaviland8602 Oh, don't worry. That date doesn't exist in most of the world. :)

[@williamarcor251]

This will be a great video to show when I'm tutoring people on mod arithmetic. Always great to see James Grimes!

[@_rlb]

Nice to see Dr. Grime again! I listened to the Numberphile podcast episode featuring him just yesterday.

[@kieronparr3403]

"It's a bit numberwang. " awesome

[@livedandletdie]

How can one not love the word numberwang. It's truly one of the best things from Mitchell and Webb.

[@teeweezeven]

Very neat that doomsday only falls on four days every new century. I thought the fact that leap years are every four years, except for every 100 years, EXCEPT for every 400 years brought complications, but in fact it made it easier

[@guteksan]

It is not really easier. The fact that you mention is embraced in the Sun/Fri/Wed/Tue pattern for 1700-2000. Normally, if you would like to count the doomsday for +100 years, it would be 100 + (100 mod 4) = 125, but since every 100 years we are 1 leap year short, it becomes 124. Then, 124 mod 7 = 5. So you should add 5 every 100 years. 2 (Tue) + 5 = 0 (Sun) mod 7 , then 5 (Fri), then 3 (Wed). But every 400 years we get this extra leap year, so now we are adding 6 mod 7. 3 (Wed) + 6 = 2 (Tue) mod 7.

[@Muhahahahaz]

@@guteksan it’s easier because after 400 years, you’ve added (or subtracted, you could say) exactly 7 days. Which is to say, the pattern repeats So 2100-24100 are literally just Sunday, Friday, Wednesday, Tuesday again. There’s no need to do any new calculations :)

[@jamisonr]

What a treat to see James Grime back. He was the reason I subscribed how ever many years ago it was!

[@liriodosvales2009]

Mike Boyd brought me here!!

[@xanthe69]

Here after Mike Boyd's vid

[@filipsperl]

It's also nice that the doomsdays work in both M/D/Y and D/M/Y format

[@7636kei]

The big nine only (anything past March), really. Then again we don't really have enough months in a year to make 3/14 ambiguous (unless we're running, idk, Mayan calendar [18 months of 20 days plus five outsiders iirc] for example?), and the mnemonics foe the Jan anchor (the one Prof. Grime spelt out, at least) also pronounces enough of the date to break ambiguities out?

[@adamschlinker972]

James was the first person i ever saw on Numberphile. Always engaging and entertaining.

[@rosiefay7283]

An appropriate day to upload, considering that 31 Oct, like 10 Oct, is a doomsday.

[@vigilantcosmicpenguin8721]

Halloween is doomsday. How fitting.

[@ed6213]

[sigh] I always get confused between Oct 31 and Dec 25. Aren't they the same? ;-) (Only old computer nerds like me need to answer. ;-)

[@lakshaymehta9399]

James: "Wednesday-third day" Joey: "u sure about that though?"🙂

[@adamcetinkent]

Who? What? When-day?

[@vigilantcosmicpenguin8721]

THURSDAY! The _third day!_

[@joeyhardin5903]

who

[@noterictalbott6102]

Great to see James again.

[@skhiozier3038]

It’s almost Christmas, 2021. That’s wild

[@jansenart0]

2021 has the same calendar as 2077, which is the year the bombs fell in Fallout, so it was weird seeing this October on the walls when I started playing Fallout 4 again this week.

[@MatDX-ej8nf]

Cyberpunk year calendar

[@oxwilder]

"It's a bit Numberwang." I love it!

[@MarcoMontaltoMonella]

This one of those videos that remind me when I initially subscribed to Numberphile! Tricks + Math + James = ❤️

[@Kevin-yn7ju]

I figured out that all 5 family members mom, dad, brother, sister, me, all of our birthdays fall on the same day of the week every year. Less than .1 percent chance of this happening.

[@flikitisideways9953]

thank you very much i look forward to find more delightful math tricks on your channel

[@lukasproper5830]

That was fascinating, I genuinely want to get good at that now.

[@yashrawat9409]

Can't have enough of Jame's Numberphile videos

[@kpk1171]

This was such an easy video to follow! I watched this video one time and then practiced for an hour and got it down. Great video guys!

[@Lep_19]

I remember in a high school psychology class we watched a video about autistic savants and some of the incredible things they can do, and one of the things the filmmakers were selling as this "extrasensory, extraordinary talent" was a young boy's ability to immediately tell you the day of the week of any given date. They presented it (as I'm sure he did to them) as some innate function in his head that understood a relationship between the date and the day without any further calculation on his part. In retrospect, how quickly he was able to calculate them still is a pretty incredible skill, but it's funny to realize that he basically fooled these filmmakers into thinking he had what was a essentially a superpower rather than just being really quick at a math trick (and by extension any audience that wasn't familiar with something like Doomsday). Certainly fooled me anyways! Great video, by the way. I tried writing up a guide on this to test my understanding, and I couldn't get anything that wasn't overly verbose and immediately confusing. The way you were able to present this such that I could learn it in an afternoon is pretty remarkable. It really isn't too tricky all told, but there's so many isolated components that are difficult to justify without a deeper understanding of the mechanics (i.e. the 12 year pattern) that it's easy to get lost in the waters. Worth it though - it's a great party trick, as you say!

[@ed6213]

Concerning speed of calculation, the late Dr. Conway (the discoverer/inventor of the algorithm) was able to calculate the day of the week for any given date in the Gregorian or Julian calendar (past or future), within two seconds. He practised by having a log-in script on his computer display a random date, for which he would calculate the DoW.

[@Lep_19]

@@ed6213 That's fascinating! Love the idea of the script, I may have to try that 😆

[@ErinnnnL]

My 3 year old son absolutely loves Numberphile. Yes, 3 years of age. Started adding and subtracting and couldn't get enough of numbers

[@namvuhoai5227]

Who else came here after Mike Boyd's video?

[@BakrAli10]

Doomsday method: 4/4 6/6 8/8 10/10 12/12 9/5 5/9 7/11 11/7 3/1 or 4/1 (leap) 28/2 or 29/2 14/3 pi 4/4 9/5 6/6 11/7 8/8 5/9 10/10 7/11 12/12 2000 = Tuesday Add the years Add the leap years (years/4) 7:31 Tips سبت = 0 أحد = 1 إثنين = 2 ثلاثاء = 3 أربعاء = 4 خميس = 5 جمعة = 6 Century: 1700 = Sunday 1800 = Friday 1900 = Wednesday 2000 = Tuesday 2100 = Sunday 2200 = Friday 2300 = Wednesday 2400 = Tuesday 9:53 Shortcuts for years: There are only 28 calendars, and then the pattern repeats every 28 years. 0, 28, 56, 84 0, 0, 0, 0 0, 12, 24, 36, 48, 60, 72, 84, 96 0, 1, 2, 3, 4, 5, 6, 7, 8

[@Piffsnow]

That's funny. A dew days ago, I was rewatching your video on singingbanana from 2008 when you presented another method to find any day after 1900. :)

[@MrSigmaSharp]

"You don't need to memorize the calendar" Gives a whole calendar worth of information to remember Nice job, professor Grime.

[@polygongaming8859]

He is right, though.

[@niklaskpunkt8335]

Mike Boyd Fam wya?

[@DavidvanDeijk]

Can we all appreciate how far Brady has come learning maths, like he really gets this and i think even considers this one easy. If you look back at the beginning of the channel that would have been so different.

[@hampuslind7546]

Normally find explanations boring but u made this one entertaining! Well done, going to try and learn this now!

[@brianh870]

I was wondering when Dr. Grime would be back! I'm only 3 seconds into the video and I'm already excited!

[@Robi2009]

Every video with Dr Grime is always cheerful and entertaining. I love his enthusiasm ☺️

[@janAlekantuwa]

I've known a similar algorithm and love using it as a party trick! My birthday also falls on Doomsday The algorithm I know for working out Doomsday grom each year is a bit different: 1. Take the last two digits. If odd, add eleven 2. Divide by two. If quotient is odd, add eleven 3. Take that number mod seven 4. Subtract from 7 5. Add the century anchor day (1700: 0, 1800: 5, 1900: 3, 2000: 2)

[@hijeffhere]

This is the one I know too!

[@SF-cq3lh]

What if it’s even? And you are talking about the year, right? (Ie the last two digits of 1776 would be 76)

[@mahiiii2112]

i have a doubt, how to know how many leap years to add 10:41.. im really confused please help me

[@inari.28]

@@SF-cq3lh in the first step, you'd just divide it by two. if it's even after that, you do nothing. e.g. for 1968: 68/2 = 34. 34 is even, so you do nothing. 34 mod 7 is 6. 7-6 is 1. 1+3 (century shift) is 4. therefore, doomsdays in 1968 were on Thursdays.

[@wyattstevens8574]

@@SF-cq3lh In at least step 1, don't do anything.

[@muuuuha]

Great video and explanation, taught myself this trick after watching your video, thanks

[@tfofurn]

I read about this calculation decades ago and could only retain the within-the-year part. I think James's mod-28 suggestion will serve me better than the original div-12. Thank you!

[@skakdosmer]

What about taking into account the transition from the now obsolete Julian calendar to the present Gregorian calendar, where several days were “lost” (which funnily enough worried a lot of people at the time), and which, by the way, happened at different times in different countries? In some countries it happened in the fifteen hundreds (I think), but in Russia it didn’t happen until the twentieth century, so the “October Revolution” actually took place in November by the Gregorian calendar.

[@lo1bo2]

Yep, I was looking for this comment. Great Britain and its colonies switched in 1752. But realistically nobody is going to ask about a date that far back.

[@Math.Bandit]

It didn't actually worry people at the time, to be fair. Matt Parker talks about it in his excellent book.

[@erik-ic3tp]

@@Math.Bandit, which book? Lost in Maths?

[@therealax6]

International time is a mess; international dates suffer from much of that mess plus the historical calendar mess. It's almost impossible to do this consistently that far back.

[@DerrickJolicoeur]

This is something I will definitely practice! I often want to know what day of the week something is on when discussing things with co-workers, and because my workplace has a zero-in zero-out policy I don't always have access to my phone. - Admittedly I could scroll through the calendar on a work computer (without internet), but it's awfully clunky

[@endrehalasz]

tell me more about this: "zero-in zero-out policy" what it exactly means

[@jodlaa5142]

@@endrehalasz I think it means something like when they get to work they have to leave their phones somewhere and they get them only when they leave, as to perhaps not leak some secret information if it's something not yet released they're working on. Basically for security in a sense

[@cbhorxo]

This is an amazing video! After watching it once, I'm now able to do the trick impeccably

[@thomaswarriner2344]

John Conway died on the 11th April 2020, a Doomsday itself. RIP Sir.

[@Goryus]

According to Wikipedia, approximately half of all known "savants" are people doing this.

[@K1lostream]

According to Wikipedia, approximately 88% of all statistics are made up on the spot.

[@rustycherkas8229]

@@K1lostream According to Wikipedia, half of all humans have "above average intelligence"... :-) (Wishin' I could meet some of them sometime...)

[@egilityx8777]

POV: you came here after mikes video

[@neongemstone_shorts]

Here❤❤❤

[@muhilan8540]

Who is Mike?

[@neongemstone_shorts]

@@muhilan8540 this video also teaching it

[@OlivierWojewodzki]

I didn't lol

[@egilityx8777]

@@OlivierWojewodzki didn’t ask

[@shiina_mahiru_9067]

I can recall that when I was a little child, I flipped over a brand-new calendar my mom bought (thinking of it as a new toy I supposed?). Then I realized: the date of the week of Jan 1 of that year and the next year (printed on the upper right of December page) was only differ by 1. Then I flipped over the old calendar, the same thing is true! I was amazed of this astonishing discovery. Then when I learn about division in 3rd grade, I realized: it was just because 365/7 has remainder 1.

[@ed6213]

Almost similar experience: I remember when my (now early 30's) son was three or four years old. We were in the kitchen and he studied a muffin tin for a couple of minutes. Then he came out with, "Dad, three times four is twelve, right?" It absolutely floored me!

[@halogenlampert]

This is amazing and I am so glad I was showed this video. I am totally picking this method up for party tricks

[@XanderNotZander]

Oh my goodness it's been so long, James actually started my forray into ASMR many years ago lol, helped my insomnia ever since, and the content is fantastic as always

[@PerspectiveInsightReviews]

I've seen people do this and i always thought it must take something special to be able to do this. But now with less than and hour of practice I can do it within 30 seconds getting it right 9/10 times

[@snowfloofcathug]

I used to know this a few years ago but have been too lazy to brush up on it recently, thanks Singingbanana

[@thorin1793]

this is so awesome! thanks for sharing

[@esquilax5563]

Nice, I remember hearing about this years ago, no idea where anymore. Maybe in _Surely You're Joking, Mr. Feynman?_ Now do one that takes into account the dates of the switch to the Gregorian Calendar in different countries ;)

[@geckogra1747]

"Monday, one day, Tuesday, two days, Wednesday, when's the day? Thurday! The third day!" Joey Tribbiani knows his stuff :)

[@LurkerPatrol5]

James got me hooked on numberphile, always happy when he's on

[@yaseenshaik67]

Amazing knowledge... Just love your videos💯💯

[@omri9325]

Finally the Hebrew way of counting the days of the week has benefits.

[@andrewchou3277]

Same in Chinese

[@pierreabbat6157]

Or Portuguese or Greek, but Portuguese says "domingo" instead of "prima-feira" and Greek says "Παρασκευη" instead of "Εκτη".

[@fred_fotch_baseball]

Same in Arabic

[@xX12VincEXx]

Came from Mike Boyd. Very well explained! I will definitely try this out when I have nothing to do 👍🏼

[@Pax1t0]

Me to

[@ignoringearthquakewarnings]

Sometimes at work, I forget what day it is. Thanks for helping me how to figure it out!

[@piguy314159]

2:11 Such a relief those dates all mirror each other so we don't have to worry about which date format to use

[@Nompass]

In Chinese, we actually call Monday through Saturday literally “Week day 1” and through to “week day 6”. Sunday is the weird cousin of the family though.

[@CanariasCanariass]

What is sunday called? Does it have any meaning?

[@m_uz1244]

what is sunday? ?? we need to know

[@Xyzerius]

@@m_uz1244 Sunday is called "Week Heaven" (星期天) or "Week-Sun" (星期日)

[@x-pilot6180]

@@m_uz1244 Sunday in Chinese: Weekday day

[@livedandletdie]

reminds me of lojban where the weekdays are also ordered. so it's 1 day 2 day 3 day 4 day 5 day 6 day 7 day... or nondei, pavdei, reldei, cibdei, vondei, mumdei, xavdei. no(0), pa(1), re(2), ci(3), vo(4), mu(5), xa(6) Sundays are either nondei or zeldei, as ze is 7.

[@DavidJSWayne]

who else got sent here by Mike Boyd!

[@rayzhang3246]

feels special watching it for the first time on Christmas

[@walt4670]

I've gotten a few in a row now each under one minute mentally, using different centuries each time. It took about an hour and ten minutes of trial and error to get to that point. Thanks for sharing this cool math trick!

[@gregb869]

Yay! Missed James Grime

[@nicosmind3]

Lee Mack is the master of naming days of the year. Seems like he cant do it, but he's a master!

[@LarsB]

This is genuinely the best and most helpful video I have seen in recent years.

[@digitalfootballer9032]

Worth a mention that "00" years are not leap years unless divisible by 400. So while the year 2000 was a leap year, 1900, 1800, etc were not.

[@JimmyLundberg]

"Look, if you need help remembering, just think of it like this: the THIRD day, alright? Monday - one day, Tuesday - two day, Wednesday - when? huh? what day? THURSDAY - the THIRD day. Okay?"

[@abheekkaushal]

You sir made my day i was looking for this

[@macronencer]

I'm really glad someone uploaded this because I used to know this trick and I forgot how to do it, mainly because I didn't practise often enough. Thanks! By the way, the only minor omission here was that you didn't warn people about most century years NOT being leap years. That only affects dates with century not divisible by 4, year ending in 00 and before March 1st of that year - but still, it's important. Did you know that there's a similar trick for knowing the phases of the moon for given dates? I used to be able to do that one as well but again, I forgot how. I seem to remember it was more complicated - perhaps unsurprising!

[@oh-totoro]

This is one thing I'm stuck on at the moment. Did I completely miss it in the video? It didn't seem to explain how we know whether a particular year is a leap year or not. And all the example dates given were easy ones from March onwards, so they didn't have to factor that in at all. If somebody gives me a date in January in the distant future of 3564 or whatever, how do I know whether the doomsday is supposed to be Jan 3rd or 4th?

[@velienne1319]

@@oh-totoro To determine if a year is a leap year or not, you have to see if it's divisible by 4, it's as simple as that. However, if the year ends with 00, it has to be divisible by 400. For example, 1700 is not divisible by 400 so it's not a leap year, but 2000 is a leap year. 3564 is divisible by 4 so it's a leap year.

[@Giofear]

@@velienne1319 exactly this. But just to make it a bit easier, if your year isn't 1700 1900 etc. and you have year like 1956 e.g. you only care about the number 56 in it when determining the leap year you only want to find out whether 56 is divisible by 4 (as the hundreds and thounsands are always divisble by 4)

[@mrjudge3722]

6

[@minaandherchaos]

This is my new favourite KZbin channel now!

[@KS-di4bn]

I knew how to find what day a certain date was in every year but I didn't know about the dates. This was really informative!

[@twincast2005]

The centuries would've been great time to remind people of the 100- and 400-year rules of leap years. I need to see this written down to memorize it, but I love number patterns, so I really should get to it.

[@FiliusPluviae]

5, 3, 2 are the smallest three prime numbers, then there's a 0. there you go.

[@LegendaryFartMaster]

James grime is always a treat!

[@sovereign3311]

Always wondered how to do this! Great video

[@ijeremyoliver]

The British arrangement of dates really threw me for a second. There was no way for me to know the 4/4, 6/6, 8/8, and 10/10 were day/month. I wasn't ready!

[@akashachuas]

From Mike Boyd's video

[@musicboy16]

Here from Mike Boyd's Channel!!

[@theohaywood4199]

Please don't ever delete this video. I think it comes in very handy , and it's a very interesting video.

[@dexobj]

As strange as this comment will sound, the "Wowee" at 1:16 made me happy! It has been so many years since I've heard someone using it. Other than that, amazing explanation! Thanks for sharing.