How to memorize the 100 year codes for calendar calculation?

(Andrew Penton) #1

I’ve only used the doomsday algorithm for calculating day of the week for calendar dates, and after searching for something more efficient I’ve found the Mental Cal app. I was wondering if there’s a system for memorizing the year codes; there are 100 of them but they all range from 0-6 so I figure there should be an easy way to do this.

Conduct lessons

Well, since I’m the one who wrote that app it seems only appropriate that I try and answer your question then. Thanks for downloading, hope the app helps you with practicing. Are you planning on competing in Memoriad or somewhere else where mental calendar calculation is a discipline?

There are two systems actually. The one I prefer to show people is one that uses seven small memory palaces; one per weekday. I say weekday because you can tread weekday and year code pretty much interchangeably. Just like 2 + 2 = 4 you could say Tuesday + Tuesday = Thursday.

So first you’d find seven palaces with seven locations each. The memory palace will represent the weekday, so find something that you can associate with it. For example, Sunday is “fun”-day… whatever that means to you. Monday is then the first day back in the office or school, so you can pick something there. Thirsty Thursday could be in some college bar or something along those lines. Basically, always a location that reminds you of a particular weekday.

After that place images for the numbers in their respective memory palaces. I’d do it as two per location. So if you got 00 = Zeus and 06 = Sushi because you’re using major system codes for your objects, then place Zeus eating Sushi in the first location of the palace that you’ve picked for Sunday.

Unlike memorizing a deck of cards, the information in these palaces will not change. So get a good interaction with the location, so that you are immediately reminded of the palace and which weekday (i.e., year code it represents) as soon as you see the image for the year. (For Monday and Wednesday you’ll need an additional location each.)

Say you have May 7, 2041 you’d look for 41 which is RT and you use “rat” as an image. You find where you placed the rat and that palace remind you of something you do on Tuesdays, so the code is 2. Actually in this example the answer to the date in Tuesday because the rest all comes out 0.

Hope this helps… you can obviously adjust as necessary and use Dominic System or whatever you use; place one image per location instead of two; etc. Main point is: where your image for the year is represents the year code by means of the palace itself.

The other approach is to use 3-digit numbers. So with the above 41, you’d use 412 = RTN… maybe “retina”. The idea here is word completion. You already have 41 from the date, so you think of the word that starts with RT_ and then chose the correct letter to complete the word.

I’m not a big fan of the second approach because it’s very restrictive, due to the fact that you’re using three instead of two letters/digits. Maybe it doesn’t bother you because you already have a 1,000 images system; however, in that case you should use the first approach and combine century and year code so that 1641 is 641, 1741 is 741, etc. Every 400 years the codes repeat, so 20xx is the same as 16xx again. That way you can skip the step of adding the century code.

Hope this makes sense. Let me know if there’s anything I should clarify.

(Andrew Penton) #3

Thanks, this helps a lot.

No, just looking to impress friends with a new skill. Maybe I will compete once I’m more competent though.


This sounds very interesting.
What is the main core purpose of this app?
What is the name of the app?

Nice to see the app creator here on the forum :slight_smile:


Thanks @Erol,

the app is called Mental Cal (App Store) and teaches how to calculate the weekday for any date in the Gregorian calendar, which is a discipline at mental calculation championships. You can find the rules here:

I initially developed it because I needed a mobile alternative to train on my commute (…super efficient me :wink: ). Non-mobile unfortunately, Memoriad didn’t have a macOS version, so that was out and Excel was a bit annoying to use because the standard date functions don’t cover the full date range needed (1600 - 2100).

After placing 1st in this discipline at the Indonesia Memory Championship in 2016, I decided to add a dynamic tutorial and release the app on the App Store, so that people who are not familiar with the algorithm can learn it too… there seemed to be a need to “demystify” the event.

The funny thing is that it’s actually easier than memorizing a deck of cards and the math required is generally obtained by the age of 10 (in most countries). However, most people insist on you having savant-like abilities even if you assure them that they can learn how to do it themselves in far under an hour.

The current world record is 140 dates per minute which I’d equate to sub-20 speed cards or sub-10 speed cubing. With the above approach (seven memory palaces) you can easily get to 10 - 20 dates per minute after very little practice (3 - 6 sec per date). By comparison, I’d consider sub-5min speed cards and sub-1min speed cubing harder to achieve for beginners.

I also added an iMessage app, so that you can play against friends and see who can get the faster times on random dates (same date per round for both). Still working on an iPad version that lets you play head-to-head in a board game fashion on the same iPad, but haven’t decided on the scoring yet (speed chess like with individual clocks running down, tennis, volleyball, etc.) I feel it gives it a nice social aspect.

So that’s basically the core purpose of the app:

  • teach newcomers how to do it in the first place
  • provide a practice tool that is convenient to use

For myself it was a good reason to dabble in some languages by translating the app into the approx. 30 languages that Apple supports on their devices. Memory palace for the weekdays, month names, etc. in those languages; plus a bit general phrases for the chat tutorial… and afterwards check with a native speaker of course. But you get to play with limited vocabulary in a bunch of languages for a couple of weeks at a time.

Currently, working on Czech and Slovak translations and after that about 4 or 5 languages left. Of course if anybody here reads this and speaks Hebrew or Japanese, please don’t hesitate to get in touch with me if you’d like to help me out with this project. :wink:

Here’s three screenshots of the app itself:

left: answer the given date by tapping the correct weekday
middle: the algorithm steps to find the weekday for a date
right: a description of the algorithm steps to perform

I didn’t include screenshots of the individual books (i.e., steps to get the codes) they mention in the chat but I could post a writeup of how to do the calculation in detail at some later point on this forum if there is an interest.

It’s basically just mnemonics for the 12 months codes and the 4 century codes, so in a way this discipline is more memory than calculation. Even more so when compared to mental square root, 8 x 8 multiplication, etc. Here you’re just adding four codes you get via mnemonics and then divide by seven… can hardly call that calculating by comparison.


Wow! I am very impressed with what you did! You’ve put a huge amount of knowledge and time into this and I applaud your efforts for this.

You mentioned about translating the app into different languages, I also speak Turkish almost as fluently as English, so if I maybe a help then you can PM me and see what we can do.

So the whole idea behind this is to find the day of the week right?

" March 23rd, 1627 ," said the committee. " Tuesday ," Bjoern said in just three seconds. "

That’s very impressive!

“I could post a write up of how to do the calculation in detail at some later point on this forum if there is an interest.”

I definitely am interested! I’d love to learn how.


That is correct.

Great, appreciate it… will do.

Okay, I’ll put that into a separate post when I get the chance. I’ll include the why as well as the how.


Lol, okay… fair enough. Shouldn’t take too long to get to speeds you can use to impress your friends once you got the seven memory palaces down. There is a list of all the years by weekday here: