The Doomsday Algorithm - Numberphile 

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

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.

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

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

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

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.

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

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

YES James is back. Mr Numberphile

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

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

Came from Mike Boyds channel

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.

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

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

That's numberwang!

1:15 Most convincing wow ever

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.

Mike Boyd bought me here

Mike Boyd Fam wya?

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! :)

Finally, the return of James Prime

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.

Mike Boyd brought me here!!

Here after Mike Boyd's vid

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!

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

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!

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.

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

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

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!

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

"It's a bit numberwang. " awesome

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

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

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

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

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

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.

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

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

"It's a bit Numberwang" made me laugh harder than it should have

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

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

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

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

Also note that years are leap years if they are divisible by 4, but not leap years if they are divisible by 100, but are leap years if they are divisible by 400.

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

Can't have enough of Jame's Numberphile videos

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.

4:55 - Only 30 days in December 5:40 - Only 30 days in March I don't know who I can trust anymore.

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)

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

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

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!

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.

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.

Thing to note is that 1900 is not a leap year, but 2000 is. It has to be divided by 400 to count as a leap year, so 1600 was a leap year and 2400 will be a leap year.

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.

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

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

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!

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. :)

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

Great to see James again.

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!

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

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

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

Actually I knew this when I was 11-12, as they teach this in India for 9th and 10th graders for a widely known Olympiad where one or two questions of this topic are asked

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

From Mike Boyd's video

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.

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

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

POV: you came here after mikes video

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 ;)

I've known about the "doomsday" moving forward every year thing for awhile now because my birthday is 10/10 and I've simply noticed this through the years so this was interesting to see

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

"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?"

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

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

This is my new favourite RU-vid channel now!

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

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

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

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

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

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!

A friend of my father tought me this trick back in 1990. One only had to memorize a row of numbers for a year - for the year 1990 it is 0,3,3,6,1,4,6,2,5,0,3,5 - these are "offset" numbers by months, 0 for January, 3 for February and so on. Then just sum the date with the offset of the month and find the remainder of the division by seven - and that's the day on the week - 1 is Monday, 2 is Tuesday (here in Russia the first of a week is Monday). Later I extended the trick by adding offset for years, keeping in mind the 28-year cycle, and do on. I still use this trick to impress people :)

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

I still don't see what 14/3 has to do with pi.

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

a calendar with sunday at the start just looks so damn weird...

In Portuguese, the days of the working week are numbered by default. Monday is the 'second day', Tuesday is the 'third day', etc... Only Saturday and Sunday have no number associated, but, because of the number system of the working week, I usually consider Saturday as the 7th day and Sunday as the 1st day.

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.

2100 is not going to be a leap year, you forgot to mention that you will have to remember which year is not a leap year that is divided by 4 (e.g. 1900, 2100, 2200, 2300, 2500... etc )

Amazing knowledge... Just love your videos💯💯

this is so awesome! thanks for sharing