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You can be a human computer too. Our cheat sheet: bit.ly/2rftqkv
Want to go NEXT LEVEL? Learn how to adjust for Julian calendar and BC dates
You might think that computers are the only things that run algorithms, but you're wrong. Here's a neat mental trick for calculating the day of the week for any day ever, developed by famous mathematician John H. Conway
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READ MORE:
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Martin Gardner, "The Universe in a Handkerchief: Lewis Carroll's Mathematical Recreations, Games, Puzzles, and Word Plays"
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It's Okay To Be Smart is hosted by Joe Hanson, Ph.D.
Director: Joe Nicolosi
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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!
It's Okay To Be Smart Hello.
It's Okay To Be Smart What about that day whensamoa skipped December 30th?
Or Russia skipping a day, because of the Gregoria’s to Julian shift?
I just ask Siri... :)
It's Okay To Be Smart 1:37
IM FEATURED IN THIS VIDEO!
I guess contributing to patreon really helps.
Before the video really gets going: "awesome i'm going to learn this!!"
5 mins in: "well nevermind..."
Here's an easy way to do that without need of remembering anything.
th-cam.com/video/KgOhcg92Fdc/w-d-xo.html
th-cam.com/video/lBjAjtWulzY/w-d-xo.html
Well in my country u have this for exams so..
@@nihaal7750 damn that’s sad
Same xDDD
*brain has left the chat*
Here's an easy way to do that without need of remembering anything.
th-cam.com/video/KgOhcg92Fdc/w-d-xo.html
* my brain
Lol
.-.
2min into the video and already confused af
MyHansSarpei don't worry, your not alone
MyHansSarpei.
Me too
2 and a half years later, I'm here, almost 3am in the moorning, I finally managed to do this on my own.
Now I just have to wait that covid disappears to use my new skill at parties. 😎
8 months later, still can’t show that party trick :’(
A year later and still no party unless you live is a specific area that let you have parties
Omg same I saw this vid in 2018 I was 10 I wanted to learn this but I didn’t understand jackshit but now after 3 years it seems soo easy
I'm from 2120, lockdown is a normal day and parties have become a historical antiquity
I don't think it's hard.. I can use this skill even I just practiced 2 hours..
Okay watching this at 3:23am was a mistake lol I'll try to learn this tomorrow.
I'm watching 2:25 am in the morning lmao
@@cloroxbleach5687 3:41 here, i think I'll save this one for tomorrow
Did that "tomorrow" ever come?
It’s no better at 2:59 AM
3:43 lol
Party tricks to impress the ladies.... this channel never ceases to amaze me.
you must get all the girls i bet
+
Mmm...yeah, tell me again about the algorithm to find out the day of the week
The girl : - Oh yeah whatever.....I'm gonna check Kevin's car, could you hold my beer a second ?
Jiraton if you're after a shallow girl, it's really your own fault
“There is a non-US version because I know we write dates funny”.
As a non-American; thank you!! I will not rest until I have learned this algorithm!
I can't find a difference between the 2 versions.
The dooms days are reversed. 7/11 - 11/7. Most of the world writes Day/Month/Year, But U.S. (and i think one other country) writes it Month/Day/Year
We write Year/Month/Day. :D
I honestly prefer Year/Month/Day ... As a programmer it makes most sense to me.
fleshTH does it make more sense because year is general, month is sorta specific, and day is specific
How to figure out the day of the week:
Definitions:
Doomsday: the day of the week that's the same for all months
1. remember:
Sunday=0 Monday=1 Tuesday=2 Wednesday=3 Thursday=4 Friday=5 Saturday=6
Doomsdays by month:
Jan: 1/3(4)* Feb: 2/28(29)* Mar: 3/14 Apr: 4/4 May: 5/9 Jun: 6/6 Jul: 7/11 Aug: 8/8 Sept: 9/5 Oct: 10/10 Nov: 11/7 Dec: 12/12
*in parathesis is number for leap year
Doomsdays by Century:
1900=3(Wednesday) 2000=2(Tuesday) 2100=0(Sunday) 2200=5(Friday)
you can add or subtract 400 (only 400) by any of these numbers and the number answer will be the same, also you can see that going in order, they increase or decrease by 100, so essentially you could go in that order too.
ex: say you were trying to find 2300. To get to one of those number up there, you would have to subtract 400. 2300-400=1900. That means 2300 also is 3(Wednesday) and so the century code is 3.
2. find which day of the week doomsday lands on (if year doesn't end in 00):
example: 2057
1. find century code (look at doomsday by century)
century code: 2
2. divide last two numbers by 12
57/12=4
3. find the remainder
57-48=9
5. divide the two answers
9/4=2
6. add together
2+4+9+2=17
7. if number is bigger than 6, subtract 7 until you can't (or divide by 7 and find remainder)
17-7-7=3 (17/7=2 remainder 3)
8. Sunday=0 Monday=1 Tuesday=2 etc.
3= Wednesday
conclusion: doomsday is Wednesday in 2057
3. How to do it? (summary sort of):
Say you wanted to find out what day it was when your parent was born. I'll use my mother as an example:
April 13, 1969 (4/13/69)
Step 1. find the doomsday
century code: 3
division: 69/12=5
remainder: 9
division again: 9/5=1
addition: 3+5+9+1=18
divide+find remainder: 18/7= 2 remainder 4
that means in 1969, all doomsdays were on a Friday.
Step 2. How to answer the rest of it:
We know that in 1969, 4/4 was a Friday. Now we count up by sevens until we get close or to 4/13.
4+7=11. 4/11 was a Friday. From now we count up.
4/12 is a Saturday
4/13 is a Sunday
That means my mother was born on a Sunday.
(By the way, I haven't found out how to do a year that ends in less than a 12 like say 2004, I could probably think it through, but I'm tired lol sorry)
But isn't 4 the code for Thursdays?
Where did you get the 4?
I don't know what to do if it's 2001 2002 2003 2004 2005 2006 2007 and up😭😭😭
Tiffany, I think you also got confused. Well, me too, but I've figured it out now. The video was confusing.
In the 2057 example, the final digits were: 2+4+9+2=17. The fourth digit "2" was the answer to "How many times can you fit 4 in 9?" This "4" is not the second digit 4 but is always constant.
So, for 1969, you should get 3+5+9+2=19, which will give you 19/7 = 2 remainder 5 = Friday.
(He actually did 1969 in one of his examples, and he got 19.)
It's still the same process. If we're trying to find the doomsday for 2002 then the four digits will be:
a) Century code: 2
b) last two digits divide by 12: 2/12=0 remainder 2
c) remainder: 2
d) remainder divided by 4: 2/4=0
e) sum of digits: 2+0+2+0=4
Therefore, Thursday is the doomsday of 2002.
For those who are/were as confused as I was: The 4 numbers that you are determining (we'll call a, b, c, & d) are respectively "a=century code", "b=how many times 12 goes into the last 2 digits of the year", "c=the remainder of the multiple of 12 from the last 2 digits of the year", and "c=how many times 4 goes into the remainder".
On a leap year - the ONLY thing that changes is the doomsday for January and February. All other math stays the same. Also it should be noted that when doing a "remainder" of a year that 12 goes into "0" times, the remained would be "x-(12x0)" where x equals your last 2 digits of the year and 12x0 is obviously 0. That being said:
1987 - Not a leap year. a+b+c+d mod 7 = Doomsday.
- a = 3, b = 7, c = (87-84 = 3), d = 0.
- 3 + 7 + 3 + 0 = 13
- 13 - 7 = 6
- Doomsday is Saturday. January 3rd is Saturday. April 4th is Saturday
2004 - Is a leap year. (Jan. and Feb. are different doomsdays, but math stays the same).
- a = 2, b = 0, c = (04-(12x0)) = 4, d = 1.
- 2 + 0 + 4 + 1 = 7
- 7 - 7 = 0
- Doomsday is a Sunday. Since it's a leap year - We use January 4th (instead of the 3rd) and February 29th (instead of 28th). All other dates will stay the same. Therefore: January 4th is a Sunday, April 4th is a Sunday, and June 8th is a Tuesday.
I really hope this helps someone out. I may make a follow up video for this!
Absolutely, this is the real formula/algorithm which needs to be clearly pointed out and emphasized in the video, not all the crap he's wasting time talking about.
You could say the "crap" he's talking about is how the algorithm came to be, how it was derived(which is interesting but it doesn't necessarily help one solve the problem), he doesn't spend nearly enough time making it clear and pointing out how to actually use it, that is what numbers we plug into it and what they mean.
In essence, he spends too much time trying to tell us what the formula is instead of showing us how to use it effectively to solve the problem.
thank you! I learned using your comment, but I am still really confused on numbers less than twelve
@@Anna-hp4zq What do you mean when you say "numbers less than twelve"? Do you mean actual year endings that are less than 12? Like if someone was born in 1911? If that's the case then 12 would go into 11 "0 times" which would make a remainder of 11. Unless you mean something else. If you clarify I would be more than happy to attempt to answer! Glad my comment helped you! :)
@@tjgrembowski Oh yeah! Makes sense if you approximate values.
Can you PLEASE explain how to determine the century code please how am I suppose to find the century code for 1600 or 2400 it makes no sense but everything else after that does
i did it after practicing it for 3 hours and watching this video countless times i am able to do it man i thought i would never do this but i can its just about patience
You also need a pretty good memory to store all those numbers
How is July 20th 2 days after a doomsday
Doomsdays repeat every seven days. The doomsday for July that is mentioned in the video is 11th. The doomsday following it is 18th of July.
This is ridiculous and amazing
I agree!
So how many more comments are u going to post?
M. Shayan about as many as it takes to get more subs than pewdiepie
my opinions are facts agreed
Here's an easy way to do that without need of remembering anything.
th-cam.com/video/KgOhcg92Fdc/w-d-xo.html
I think I'll go to play something more on my intellectual level. Anyone want to join me in a Hungry Hungry Hippos game?
JuQmadrid
Naw, too hard for me.
Sign me up!
Lmao
Maybe you can just play fortnite 😅😅
Well nevermind
I thought it was meant to be easy
Just put it on your finger.
does not make it easy
I thought so too...but it just clicks after a while. Do it on paper first.
it is as easy as it gets~
although still complicated but as easy as it can be
You still have to practice it. It won't be easy until you've made the structures in your brain, that's how learning works...
"Congratulations, you just used an algorithm!" Uh no... You did...
LOLOLOL sad
Doing the arithmetic = using the algorithm!
Guys, I barely have words to describe what I am feeling right now. I`m so excited!!! I have just talked to a bunch of friends finding out the day of the week of their date of birth. Thank you so much and congratulations for this awesome video.
You assume I can do division in my head...
"I don't know. Get a calculator." 👦 andhisnameisMarcoDiaz
same i am just young
@French toast Rat You're probably aren't young anymore; you should try to learn it now
@@frenchtoastrat5170 you're definitely not young anymore; you should try to learn it now
No! This is _my_ thing! What will be my party trick now? ...just kidding, I'd never be invited to a party.
maybe you will if you can do this
@@oliverdiamond6594 four years later, I'm so rusty I've basically forgotten how to do it, and yet still no party invites...maybe we're both onto something?
Took me about an hour, but I feel so cool and I'm definitely going to show this off to my friends!
Rob H. Haha that'd be tough. My friends are all super smart though so they'll probably think it's cool
Rob H. Hmm I'll just have to specify that they should choose a date in the Gregorian calendar.
Om Pandya I got Friday as well.... I'm not sure why.
Om Pandya you are doing a mistake in finding the remainder. Dividing 5 by 12 gives a remainder 5 not 0.
k1w1 they can look through the calendar on their phones
It finally clicked after about an hour of focusing on how to do it. Your leap year number with the thumb confused me, but i realiszd later you were thinking ahead and saving the step of going through the days in your head. Impressive shortcut but it confused me as a student. The cheatsheet was insanely helpful to memorize the steps. I've practiced this for probably 7 hours over the past 2 days to get quicker at it, and I can't wait to do nothing with this skill. :) Thank you.
For those still confused here’s how you do it (I used international dating method btw):
First look at their description and learn the 3 rules - weekdays, doomsday by month and doomsday by century.
Once you know these 3 rules follow these 6 steps: I will use the example 3 / 5 / 2020
1) Find the century code (by using doomsday by century) = 2020 is 2000 so Tuesday with a value of 2 (keep this value on your first finger).
2) Divide the last 2 digits of the year by 12 (divide by number of months in a year) = 20 / 12 is equal to 1 remainder 8 (keep these 2 values on your middle and third fingers).
3) Divide the remainder of the previous step by 4 (divide by number of times there’s a leap year) = 8/4 = 2 (keep this number on your pinky).
4) Add up all the numbers that you calculated = 2+1+8+2 which equals 13
5) Divide 13 by 7 (days in a week) and the remainder gives you the day of the doomsday = 13/7 = 1 remainder 6 so the code for 6 is Saturday so the doomsday for May is Saturday.
6) Work from the doomsday to the day you want to know = May is the 5th month so the 9th of may is the doomsday (doomsday by month) therefore work from the 9th (which is a Saturday) to the 3rd which means that the 3rd is a Sunday!
Tip: remember to watch out for leap years, you can tell a year is a leap year if the year is an even number and the sum of the individual numbers equals a number divisible by 4, e.g: 2020 is a leap year as 2020 is an even number and 2+0+2+0 = 4 which is divisible by 4 so it’s a leap year. Leap years only afffect calculations for jan and feb so don’t worry about it for other months.
Here's another tip i recently discovered, step 5) is incorrect, the correct way to do this is to subtract your answer to step 4) by 7 (13-7) but do this in a modulus as it can give a different Result (example if calculating 2003 you dont leave it as 2 - 7 =0, you do the modulus to = 5)
Good tips and it helped me to understand more about the technique, however leap year calculation is wrong here. You cannot sum up all numbers together to define whether it is a leap year. e.g. 2016 is a leap year but 2+0+1+6 = 9 is not divisible by 4. Leap years are those years that can be divided by 4 with exception when it is divisible by 100, it has to be divisible by 400 as well to be counted as the leap years. e.g. 1500, 1700, 1800, 1900 are not leap years but 1600 is leap year. Detailed algorithm as follows:
if (year is not divisible by 4) then (it is a common year)
else if (year is not divisible by 100) then (it is a leap year)
else if (year is not divisible by 400) then (it is a common year)
else (it is a leap year)
@HB2803 then the sum in step 4 will be equal to the remainder / modulus if divided by 7.. so in step 5 result will be same as step 4 result
How do you use that algorithm for a year with the 2 last digits smaller then 12, for exemple 2007??
@@renanpigozzo5728 for 2007.. 20th century is 2, 7 divided by 12 = 0, remainder is 7, 7 divided by 4 = 1 (with remainder)
So for 2007 doomsday will be remainder of (2 + 0 + 7 + 1)/ 7 = 10 / 7 = 1 remainder 3. Therefore 2007 doomsday is 3 = wednesday
Wow, I understood absolutely nothing at all!! Still, great video. Somehow, interesting.
Same here...
That's incredible! You lost me after numbering the days of the week though
Oh, so that's how the dude at TED did it.
I was thinking about the same thing😂
Same
video link?
Wut
I came here after watching it cuz i knew there must be a way on yt how to do it
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.
😂😂😂😂
1:33 my brain🤯! My damn head literally starting hurting trying to figure this out
Me: 10.19.1902 *doing math* oh, Friday!
Joe: It was a Sunday.
Me: oh..
You stopped at the Doomsday, without adjusting for the date of the month.
P.S. Reply after a year lol, IDK if you already had that figured out
ChaltonDi Lets try my friend’s birthday
March 15, 2005
Century code = 2
5 / 12 = 0
Remainder = 5
5 / 4 = 1
2 + 0 + 5 + 1 = 8
8 - 7 = 1
1 = Monday
Doomsdays of 2005 = Monday
Doomsday of March = March 14 (March 7, March 21, March 28 are too)
March 14 = Monday
March 15 = Tuesday
March 15, 2005 = Tuesday
-------
Shortcut:
Century code = 2
5 / 12 = 0
Remainder = 5
5 / 4 = 1
Adjustment = 1
2 + 0 + 5 + 1 + 1 = 9
9 - 7 = 2
2 = Tuesday
March 15, 2005 = Tuesday
Who’s joe
No problem it was too close
@@DanksterPaws why you used the number 4 in the 5/4 step?
This is the very first video in TH-cam that I keep rewinding a lot and rewatch so many times.
And I'm so obsessed with it.
First, assign each day of the week a number. (01:13)
*Sunday - Saturday* are *0-6* .
***
Next, you’ll need to remember the specific dates that are always the same day in any given year. (01:25)
*4/4* , *6/6* , *8/8* , *10/10* and *12/12* are easy to remember.
*5/9* and *9/5* are the same date flipped.
The same works for *7/11* and *11/7* .
The others will just need to be remembered. It helps to apply a pneumonic or useful piece of information about that day to help remember.
*03rd January* ( *_4th_*_ on a leap year_ ),
*Last day of February* ( *_28th_*_ or _*_29th_* )
and *14th March* ( _Pi day_ )
***
You’ll also need to remember the century codes. These follow a simple pattern, which will help. (03:03)
I’ll start with _1500_ , but this pattern should work no matter how far back or forward you go.
*1500* = _Wednesday_
*1600* = _Tuesday_
*1700* = _Sunday_
*1800* = _Friday_
*1900* = _Wednesday_
*2000* = _Tuesday_
..etc
You just need to remember Wednesday, Tuesday, Sunday, Friday and at least one of the centuries (Like *2000* = _Tuesday_ ).
The more you do this trick, the easier this part will become, as you should find it easier to remember more of these.
As mentioned in the video, the Gregorian calendar repeats every *400* years. So *1100, 1500, 1900, 2300* are all Wednesdays (as shown in the WTSF cycle)
***
Lastly, you’ll need to know if the year is a leap year or not as it will affect your end result if your given date is in January or February. Fortunately, there is a simple trick for this too.
1. If the _last two digits_ of the year are _evenly divisible by _*_4_* , it is a leap year.
2. The exception to this rule is if the whole year is also _evenly divisible by _*_100_* . (Not a leap year)
3. And the exception to that rule is if the whole year is also _evenly divisible by _*_400_* . (It is a leap year)
(Side note: Only paying attention to last two digits of a number to see if it is evenly divisible by 4, works for any number. It doesn’t just work for this leap year trick)
***
Now you have this information, you’ll need to know how to apply it to any given date of any year. (02:26)
Start by working out how many days after a doomsday your date is.
*19th October?* _9_ days after *10/10* (or a week and *2* days). Remember that number (It would be *2* in this case. Divide by 7 and keep the remainder)
Next, work out the century code and remember the corresponding number that goes with that day.
Now comes the arithmetic.
Take the last two numbers of the year and divide them by _12_ . You’ll need to remember how many times 12 goes into it along with the remainder.
Divide that remainder by _4_ and remember that number also (Ignoring remainders this time around).
***
Now add all those numbers together ( _Days after Doomsday, Century code, The number you got for dividing by 12, the remainder for that and the number you got for dividing that by 4_ )
Divide that total by *7* and focus on the remainder. The day that corresponds with that number (the very first step) is your final result.
***
Here are some examples to practice: 00:10, 00:55, 05:06, 05:27, 06:59, 07:13.
Also, these three explained ones: 03:33, 03:55, 04:16
***
I hope this helps anyone who was feeling a little confused after watching this video.
*Sunday-Saturday
Your week only has 6 days!
I thought Saturday was the most important day, but that's just me I guess.
Carl-Michael Thanks for the instructions
@@buddyclem7328 - Hm, I didn't get any notifications for replies on this comment. Thanks for pointing out that mistake, I've fixed it now. :)
This made me remember those fix dates better. Love the pi day
Wait but... I just checked a date and you don’t need to find the days after doomsday and divide it by 7 and then add it back, it gives you the wrong day. I could be wrong but I didn’t need it what I did
I got it on my first try before the end of your first example. So, what, it takes approximately three minutes to learn? Super easy, but robust. It feels like a super-power!
I was trying to figure this out in my head yesterday... today it’s in my recommended. 🤔
brain nanobots for sure
th-cam.com/video/lBjAjtWulzY/w-d-xo.html
I dont get it 😭😭😭
Stacy Stroker same
Me neither
Faisal Al-Bandar I just binged watch a bunch of AI videos....my mind is kinda clogged rn :/
I don't either. What the heck is a doomsday? How does knowing a doomsday help you know other dates? What the heck is the formula? How do you figure out the century number???
Andrea B I'll google all that later....probably share my findings here. But for now, hmwk :(
Now I understand why Google was created :D
Tomorrow I've got an exam! I can now find weekdays of any date in 30-45secs. Thank you so much!! You're awesome!!!!
Who is here from Mark Rober
SAME
Yup
yep
Me
Me lol
Who else is here with no hope? 😂
George Elsham watch it a few times, do some independent research (google 😜), and you'll start to get it....trust me, I've been at it since a few hours after the vid was uploaded
George Elsham me
George_E lmao! this is the comment i’ve been scrolling for
Nah I remember being able to do this about a year ago but I can't do it now because I completely forgot how to do it this explanation is really poor and overly complicated
Next parties will be fun... Oh wait I don't get invited to parties...
francisco chinchilla resende hehe... Why don't you organize one and show it to them all :-)
Yeah, u lost me
Anyone else hate dividing by 12, finding the remainder, finding how many 4s fit into the remainder and then adding them all together? Yeah, me too, so that's why I devised a simple trick that's a little bare bones at the beginning but it'll help you calculate the day of the week in just a few seconds. So here it is:
I want you to recognize that there are 3 main elements that help you figure out the day of the week for any given date: the day offset, the century offset, and the last-two-digits-of-the-year or LTD offset (sorry, I couldn't think of a better name haha).
So the formula for the day of the week for any given date is:
= x + y + z where
x = day offset
y = century offset
z = LTD offset
All three values have a range of 0-6, so when figuring out the day of the week, you just have to add those three small digits together!
Okay. Let's begin with the first part of the equation: the day offset. The day offset is basically the number of days a given date is away from the nearest doomsday before it. For example, 7th Apr is THREE days away from 4th April, a doomsday. So the day offset of 7th Apr is 3. See? It's that easy!
However, how about 26th April? Obviously, you're going to have to divide 26 by 7, take the remainder and then calculate how many days away it is from 4th April, HOWEVER we are NOT doing that. The math's too complicated to do in your head. An easier way to do this is to memorize ALL the possible doomsdays by HEART. That is:
Jan 3 10 17 24 31 (+1 if L.Y.)
Feb 7 14 21 28 (+1 if L.Y.)
Mar 7 14 21 28
Apr 4 11 18 25
May 2 9 16 23 30
Jun 6 13 20 27
Jul 4 11 18 25
Aug 1 8 15 22 29
Sep 5 12 19 26
Oct 3 10 17 24 31
Nov 7 14 21 28
Dec 5 12 19 26
If you want to find out what the day offset for 19th Aug is, just subtract 15 from 19 to get 4-->the day offset! It's also helpful to memorize which months have the same doomsdays in both leap and non-leap years. For example, 5th September for any given year is the same day as 5th December in the same year. There are quite a few more month pairs/triples; I'll have you figure out those months yourselves.
Now, let's go to the second part of the equation: the century offset. This one is already stated in the video. For example, if the year is 2072, just take the first two digits of the year and take its corresponding century code (in this case, the century 2000 has a code of 2). That's our second value. If you want to know how I got this, just refer to the Doomsday by Century table at 3:21. The century code repeats every 400 years, so if you want to find out what's the century code for the year 3281, just keep on subtracting 400 until you end up with a century with a century code that you already know.
The third and quite the most complicated part of the equation: the LTD offset. This the number you get from dividing by 12, getting the remainder... blah blah blah. This is where most people give up trying to learn this stuff, so we're NOT gonna do this. Instead, try to memorize by HEART these LTD codes for these leap years:
00 - 0
04 - 5
08 - 3
12 - 1
16 - 6
20 - 4
24 - 2
28 - 0
32 - 5
36 - 3
40 - 1
44 - 6
48 - 4
52 - 2
56 - 0
60 - 5
64 - 3
68 - 1
72 - 6
76 - 4
80 - 2
84 - 0
88 - 5
92 - 3
96 - 1
I know. Memorizing sucks. But it definitely gets easier with practice. But did you notice how the patterns for these codes go 0,5,3,1,6,4,2 and then over and over again? You can use this pattern to memorize these codes easier!
Alright, so for those who are still reading this, what's the LTD code for 1932? Well, since the last two digits of 1932 is, obviously, 32, we just take the LTD code for 32--> 5! That's the third value!
But how about a non-leap year like 2163? Well, just like the day offset, you take the LTD code of the most recent leap year before the given year: 2160 is a leap year, and 60 has an LTD code of 5. But wait, we're not stopping here yet. We first need to count how many years away 63 is from 60, and then add that to the 5. So 63-60 = 3, and then 5 + 3 = 8. By taking away 7 from 8 (you can take any amount of 7s from your sum), we get 1--> the LTD code for 63!
Now that we know how to compute for all three parts of the equation, we're finally ready to calculate the day of the week for any given date! Here's an example:
18th June, 1990.
x = 18 - 13 [Jun 13 is a doomsday] = 5
y = [the code for 1900] = 3
z = 5 [from 88] + 2 [90 - 88] = 7 - 7 = 0
Add 5 + 3 + 0 all together and we get 8 - 7 = 1 (Monday!)
Here's another one:
21st February, 2068
x = 21 - 15 [Leap year, so Jan and Feb doomsdays adjust 1 day. Feb 15 is a doomsday] = 6
y = 2
z = 1 [from 68]
Add 6 + 2 + 1 and we get 9 - 7 = 2 (Tuesday!)
And there you have it! A quicker way to find the day of the week for any given date!
Sarcadistic This is way harder than just using basic arithmetic to know what a “remainder” is
It’s literally the first way we’re told how to divide and it happens in first grade.
Oh 12 can’t fit in 8? Well thats too bad I guess it fits in 0 times and leaves 8 behind. Tadaaaa 8 is your remainder and you dont need any fancy stuffs
I thought this was going to be an easy trick like using your knuckles to find out which months have less than 31 days
I don't know how to do that
@@GRIMPONG well now you do
Kenny Arias no they don’t cuz no one explained it
@@donutlover7728 they have google
why arent we learning this in school ?
because school is dumb
Julia Taylor the educational system is fucked
Because is useless
Learned this in my high school computer science class 40 years ago.
*Good Question* Julia
Because school is meant to make an obedient to authority, productive working for money wage slave out of you. (Most of us) Unless you go to the "right" school. Money?
If humans can do this they do not need to buy stuff to tell them, and because it is convenient and humans like to be convenient. It saves brain energy. But if you don't use it you lose it. Paradoxes of life.
AI that uses algorithms like these. Because smart people have a lot more of these brain hacks up their sleeves.
For memory and stuff like that.
*Mentalists* like Derek for example.
Do you know the history of education?
Do you know many things taught in educational systems are less true.
- Columbus and USA for example.
- The human history for example.
- Where is the cradle of life and the cradle of civilization? Africa and Middle East..
Not caveman, stone age and Egyptian civilization - Greek - Roman.
What about the Sfinx? They are Mesopotamian /Sumerian.
- E.T. life forms? Panspermia?
- Caveman / Neanderthaler extinct? Yet there are Europeans with Neanderthaler DNA. So intermixed.
- Dinosaurs extinct, yet we have great crocodiles, white sharks etc. Less oxygen in the Air so Smaller life forms. Fact. And we still have massive beasts living amongst us. Blue whale for example.
Educational systems clogged up with old, incomplete and untrue information.
Il really love that there is an international version of the cheat sheet, it really matters :)
Ritter Joe there is in the description. Click the link it has both
_For clarification:_
4:04: "July 20 is two days after a doomsday...." It's actually _nine_ days after a doomsday (7/11), but since 9 mod 7 = 2 mod 7, it doesn't matter.
4:25: It should say "Century code - 3," not 0, since he adds 3 on his index finger, and according to the diagram at 3:20
For clarification 7/18 is also a doomsday, as is 10/17 and 04/11. The ones shown in the video are just the easiest to remember.
doomsday plus 7, is also a doomsday. basically you have 7/11, 7/18, 7/25, are on the same days. so 7/20 is in fact 2 days away from 7/18, which is a doomsday
Gosh hahaha my brain ain't grasping anything hahahaha maybe too much coffee? Hahahahaha sigh.... I need to sleep
Nice
*HELP!!! What do you do if four fingers add up to 6 or less?* 2:49
4:03 *And in his original instructions he failed to clearly explain when to use the thumb.*
*So what IS the "Rule of Thumb"???* 🤣
Whoa... this is much simpler than calculating the odd days to find the day of the week. Thanks for sharing this method!
This reminds me of a pattern I found in my school's bell schedule that I used to calculate the bell schedule with almost perfect accuracy until I had the bell schedule memorized...
Hey Joe, fantastic stuff! I knew about Zeller's algorithm before but that's quite a beast to memorise. This is much simpler. I took inspiration from this video and tweaked the algorithm to help me memorise it.
First, I tried to keep the date format consisted (4/4, 6/6, etc.) for the odd-numbered months to get 0/1, 3/3, 5/5, 7/7, 8/9, 10/11 (because July and August both having 31 messes it up a bit, and 0th January actually behaves), with the caveat that you have an additional 3 to the day total if you're using an odd-month.
Secondly, @2:53 instead of dividing the year by 12 and subtracting, I found that taking the year modulo 7 and adding the divide by 4 mod 7 is sufficient.
Overall my tweaks result in: day = Doomsday from year (0,2,3,5) + Year mod 7 + floor(year/4) mod 7 + (3 mod 7, only for odd-months) + T-X which is the difference between the date in question and the Doomsday date for the month.
Thanks for the inspiration to spend a morning playing around with algorithms and maths :D
Excellent work!
4:16 Hey! It's my favorite Ghostbuster! *Runs to bomb shelter*
I was put off by the US date notation, mm-dd-yyyy
But cool video!
To tell you the truth I do not like that much our (european) dd/mm/yyyy notation, because it does not go well with time (hh:mm:ss). I switched to yyyy-mm-dd ISO format and it is very handy and I can avoid confusion easily.
NetAndyCz it sure sounds interesting! It might very well work. However, for example, in Dutch I'd say in a conversation "five April 2018" so dd-mm-yyyy in writing would be easiest, in some languages
Yeah, but languages are sometimes stupid I mean 97 in French is literally four times twenty plus ten plus seven. However for writing purposes (and ordering of files and data) it is good to use systems with some logic to them. And maybe if they are used people will change the way how they talk, though I am not sure (French is super silly when it comes to numbers 17-19 and 80-99)
both dd-mm-yyyy and yyyy-mm-dd are fine and make sense, but mm-dd-yyyy is nonsense.
yyyy-mm-dd is better for ordering and dd-mm-yyyy presents the usually most relevant information, the day, first, since the year and even the month can be inferred in most conversations.
yyyy-mm-dd is the best for chronologically ordering as it presents info in decreasing size order, and fits best with the way we measure smaller amounts of time (hh:mm:ss)
dd-mm-yyyy is better for colloquial speech as it presents information in order of importance / relevance.
mm-dd-yyyy has no redeeming qualities honestly.
Scared me and excited me at the same time! God bless your channel!
The last part of 'seeing' the remainder of dividing by 7, was made so much simpler by just thinking in terms of modulus 7. It also helps shift up and down from the doomsday since you can add or subtract mod 7
but what happens with the 10 missing days of 1752, it would change the days of before that by 3
Barnesrino Kripperino ??????
I was wondering that too but I think it’s only for the English calendar
Well then, you take the 3 days difference into account and you adjust your result...it’s only one more instruction added to your algorithms.
Unknown Unknown he smart
@k1w1 *Magna Carta
What's wrong with the middle finger in that thumbnail!
Fun fact: My fingers are crooked as heck, I played goalkeeper in soccer for like 20 years of my life! ⚽️👐
It's Okay To Be Smart soccer? Science nerds care not for footsports 😩
It's Okay To Be Smart My finɡers are naturally crooked, and I am underweiɡht, so my friends often call me 'spider-finɡers'
Thankfully you didn't see my fingers
It's Okay To Be Smart what is a sock car
The video got many people confused because of the "4" in the first (2057) example.
2057 gives: 2+4+9+2=17, with the fourth digit (2) being the answer to the question "How many times can you fit 4 in the remainder (9)?" This "4" is not related to the second digit 4. It is constant, and will always be 4 each time you compute.
Hence, for the 1776 example, we get: 0+6+4+1=11, with 1 being the answer to "How many times can you fit 4 in 4?"
And that's also why for 1969: 3+5+9+2=19.
Hope this clears the confusion for everybody. I wish they hadn't used 2057 as the first example. Took me a looong time to figure it out.
UPDATE: For 7.13.2305 (the problem he first solved "mentally"): 19th century is 3 (Wednesday), so:
Century Code: 3
5 divided by 12: 0
Remainder: 5
How many 4 in 5?: 1
---------------------------------
3+0+5+1=9, 9/7=1 remainder 2 = Tuesday. So, 7.11 is Tuesday, 7.13 is Thursday.
killjoyxvi
Thanks a lot !!!!!!
@@Highlyghts how to calculate century code??
How to calculate century code?
@@vipinsl if you look at the diagram at 03:20 you'll see the century's codes repeat every four years, so just remember the columns are 3, 2, 0, 5 and work out which column you need. I personally just remember 1900 is in the 3 column, I then count up or down rotating through those numbered columns.
Also, don't bother remembering the day they allocate to the columns in that diagram because it's just confusing. You'll still add other numbers to the century code so it'll probably end up changing. I don't know why they put those days in there, it's misleading.
@@lank_asif yeah, i got it
1:14 when you're a programmer and you start counting
I watched this video a few times and gave up on it. A week passed and someone said a date and I remembered almost every step along the way. I watched the video again today and I got it easily. If anyone is struggling maybe sleep on it, it’ll come to you
why showing dates as month-day-year?? it's so confusing !
I know I never get the right dates when watching the American News, they work backwards from the rest of the ENTIRE WORLD, this is why nobody understands that 9/11 in September and not in November, I live in Canada and we have half of Anglo Canadians going with the American System and the other half going with the International system and the French Canadian going International System, it is a Mind F*&k, some people get to a meeting in different month no kidding.
Because in their system water freezes at 32 and boils at 212 degrees!
@@vladchira521 joke's on you. You cant have 4/20/69 in your method
@@acarriere8534 Actually the rest of the world isn't all day month year. In Japan it's year month day, and that's actually the new international standard. It also makes the most sense to put the year first just as we put the hour first when talking about the time. Writing the date as day month year is like writing the time as second minute hour.
right
Really, this video is amazing. You explain a concept and give us something to do at home. Something fun and something we can amaze our friends with. If possible, make more video in this way. I love it!
OMGolly I'm going to watch this 20 times to figure out how to do this during partys
Man, I can’t even do simple addition in my mind. I always count with my hands 😩
Very good system. I am an average person and took me several hours to get it so dont be discouraged.
This is incredible, and a lot more simple than I thought it is! I never thought I would be able to learn such a thing like calculating days (seing other people just do this seemed impossible to me...) 😂
I always loved your videos since the beginning of this channel, they all have something to them, that makes me "stay curious" every single time.
Your explanation on this was as perfect as it could be. I was able to follow every single step. It took me a bit to learn it, but now I'm able to do this like you in a few moments! :)
I'm so happy about this, because riddles and quirky mathematical stuff is just my thing. (And this is actually a pretty cool trick you can show your family and friends!)
It's rare that I comment on videos, english is not my native language and it takes a little while to write a comment, but I just felt like I needed to on this video. 😄
*Thank you very much for sharing this with us!* :)
Are you still able to do this trick 3 years later? Ur English is great btw
@@kasenpatel Duh, you can do this trick in 300 years. Or even 3000 years later. Given we’re still using the Gregorian calendar
@@DanksterPaws dUh... he's asking if he still remembers how to do the trick🗿
@@maazali1595 Gonna be honest with you, that was one reply of many, many, many that I made in response to some very obvious questions. I guess I was carrying a bias while replying.
I have three exams this so I should probably start studying..........
or I'll just learn how to figure out the day of the week for any day ever.
I don’t get it :/
Are you putting the numbers on your finger? You have to put the numbers on your fingers.
I just replayed the video twice and viewed the cheatsheet and now I'm able to calculate the day for any date ever! Thanks a ton :)
Thank you kindly, good sir! I always wanted to learn how to calculate this. 👍👍👍👍👍
how do you get the century code
And I thought my fingers were useless!
Wait, no that was my brain.
That's way too many things to remember man, i could just look at my cellphone, it is great info tho.
RoarOfDamnation that’s actually quite a lot slower. If could got fast and could do this in 5 seconds something then it would be several times faster
Yes but that's assuming you use this random skill constantly, and even then, even if you do, you would still have to go to a computer anyways and annotate the future date and input it into a schedule, and if you are making historic research the same, you will need to end doing it with a computer.
For every other case you will lose the skill by the end of this year, is just a nice trick.
thanks for the trick.. once you know how to solve, it comes handy. Especially when you are trying to fig. out the day of any dates in the current year.
after doing one date as practice I was able to do it confidently
this is pretty neat
How do you figure out the century code
ryanpetras 1225 uidk
I guess just memorize, or count backwards?
Easy. You just put it on your finger.
Working out the century code requires memorising them. Fortunately, I realised there is an easy pattern.
The century code always cycles between Wednesday, Tuesday, Sunday, Friday. The video’s earliest century code starts with 1500, so I will too.
1500 = Wednesday
1600 = Tuesday
1700 = Sunday
1800 = Friday
1900 = Wednesday
2000 = Tuesday
...etc
To work out if a year is a leap year:
If the year is evenly divisible by 4, it’s a leap year.
The exception to this rule is if it is evenly divisible by 100 also.
And the exception to that rule is if it is also evenly divisible by 400, it is actually a leap year.
A quick way to work out if a number is divisible by 4? Work out if the final two digits are divisible by 4.
19[69]? No.
19[84]? Yes.
21’8[04]. Yes.
Took me about 2 hours, finally did it :)
Version Omega same, I feel so cool now!!
bailal1 I wished that I could be like the cool kids... is this how they feel?
Version Omega lol probably not
Can u tell me how to figure out the 2001 or 2002 dates
Ethan Shepherd with anything that close just count from the doomsday of 2000. 2001 is a Wednesday and 2002 is a Thursday.
did anyone come from that mark rober video?
Me!
I don't think anyone knows how incredible this is
My algorithm for it:
1. Take phone out of pocket
2. Unlock phone
3. Open calendar app
4. Scroll to date asked for
5. Read answer
Much easier to remember
Great work!
One issue: at 4:18 The century code diplayed should be 3, not 0.
:)
Allan Braga how do we calculate century code
Prat memorization of 4 of them, doesn't matter which 4 centuries you do as long as their in order so let's say
1900, 2000, 2100, and 2200.
Since our calender repeats every so often, (400 years) it's a simple conversoon. Century code for 1500 and 1900 are the same. 2200 and 3000 are the same, and this is because the difference is divisble by 400.
Thank you!! I was scrolling down the comments to see if this was the case!! This initially messed up my understanding of the algorithm and I thought that leap year century codes defaulted to 0 regardless of the century and I couldn't explain why I could solve any dates BUT yeap year dates!! It was because I kept using 0 as my century code! Ahhhggg 😂
Anyone here from Mark Rober's vid
Which one
1:37
IM FEATURED IN THIS VIDEO!
I guess contributing to patreon really helps.
Pi What?
pi day
i contributed to patreon
He didn’t mention how to get the century code. 2:28. How do we know 2000s doomsday was a Tuesday?
It's just a pattern you have to remember, Tuesday, Wednesday, Friday, Sunday if you're going backwards, and if you're going forwards, reverse it, so the year 2100 is a Sunday, that pattern will repeat every 400 years in either direction
I found using odd-days concept much more easier than this, this confused the hell out of me. Good job everyone who got it
Not only you write dates funny, you write speed, weight, length, temperatures, volume too, funny as hell.
@@aaronwestley3239 its how you write dates out in words. march 4rd, 1983 is 3/4/83. its the exact same order as you write it in words
How is July 20th 2 days after a doomsday
Am I stupid for not being able to follow this on the first try?
Osik Brodsky i stopped at one...a tuesday...
Same
Thanks
No its pretty complicated.
Computers can do this sort of thing with ease, and computers are really dumb. Being good at math and memorization doesn't make you smart. There is more to intelligence than that.
Thank you! I took me sometime to understand (typical math difficulty) yet I managed to "manipulate" lol. This is a great help for my upcoming board exam.
-keep sharing knowledge. Appreciating here
How is July 20th 2 days after a doomsday
@@colbymaxwell244 Because doomsday of july is 11th of july, which means 18th of july (11+7) will also be a doomsday, which means 20th of july will be 2 days after that years doomsday.
YES! YES! I FINALLY GOT IT AND I FEEL SO ACCOMPLISHED!!!
Algebra doesn't come from Al-Khwarazmi (whicl algorithmus - how he was called in Greek - comes from). Algebra comes from "al-jabr", which was part of name if the book Kwarazmi wrote and the method he proposed "Al-jabr w'al-moqabala", meaning "Completion and Balancing" ("The Compendious Book on Calculation by Completion and Balancing - Wikipedia" en.m.wikipedia.org/wiki/The_Compendious_Book_on_Calculation_by_Completion_and_Balancing) and is Arabic.
I thought this was going to be so easy but nope😔
Here from Mark Rober
Same ☠️
There's a typo at 1:31
IguanaSloth can't believe you :D
April 4th - Wenesday.
That had to be on purpose.
Nice catch though.
Woaw
IguanaSloth i noticed that too
One addition to this method, as every year is a leap year if it is divisible by 4 similarly every century lies in a leap year if it is divisible by 400, what i mean is 1800,1900 are not leap years but 2000 is a leap year and the next leap year is 2400.
Hey
What if the year is 2002?
How will u divide 02 by 12?
well the century code is 2 so:
2 / 12 = 0 remainder 2
so this means currently we have 2 on our index finger, 0 on our middle finger and 2 on our third finger.
now divide 2 by 4
2 / 4 = 0
so in total we have 2,0,2,0
2+0+2+0 = 4
4 is a thursday and if you look it up that is the correct doomsday for 2002.
Just use the same method
Thanks! I never knew about Conway's method - you're right, it's easier than Lewis Carroll's. I'd make one adjustment though - the century code table makes no sense to me because the columns from left to right go 3, 0, 1, 2 (mod 4). I would put the centuries divisible by 4 in the left column, making it a simple matter of memorising 2, 0, 5, 3. If you know Carroll's number-mnemonic trick, you could translate this to a word (I've used aNSeLM - as in Saint Anselm, which sounds kind of historical so fits nicely).
A couple more things: first, as others have noted, you have to watch out for the Julian calendar, and the fact that it changed at different times in different countries! Second, did you know there's also an algorithm for working out moon phases? I used to know it but I've forgotten it now. I may go and re-learn it. Thanks for the inspiration!
Who is here after Mark Rober's video?
Haha, April 5th 2063 is First contact Day!🖖🏽
That's so complicated. I even had to google what a remainder was because I never learnt in school english mathematics vocabulary, and I never thought I would use it in my life anyway
Cool Trick
helped me a lot
How do you know July 20th is 2 days after doomsday ? And being French, having all the dates written backwards makes it even harder. Good vid btw. ;)
For the 7.20.1969 example: We need to know the day the doomsday is on. Since for July the doomsday is on 11th, even if we add 7 days (7/18) the day of the week will be the same as on doomsday. 7/20 is two days later.
Il y a une fiche internationale dans la description
If 7/11 (Jul 11) is a day, Jul 18 is the same day, a week later. So the 20th is (one complete week ) + 2 days.
:)
How do you find the century code? For 2057 it is 2 and for 1776 it is 0. I don't get it.
It's been figured out for you. You memorize it. It's a simple repeating pattern of four days and numbers as shown in the chart:
1500 = Wednesday = 3
1600 = Tuesday = 2
1700 = Sunday = 0
1800 = Friday = 5
1900 = Wednesday = 3
2000 = Tuesday = 2
2100 = Sunday = 0
2200 = Friday = 5
. . .
and so on.
@@TheRealLaughingGravy At 4:22 in the video it says the century code is 0 but he says 3. No wonder people are confused
>knows everything about dates
>still can't get one
took me about an hour but I got it down pat thank you for being awesome
Nothing to fear... you know the right thing. Illuminated.