Show HN: Weekle – a web app to learn how to calculate the day of the week (opens in new tab)

(benjoffe.com)

122 pointsbenjoffe2y ago32 comments

Mentally calculating the day-of-the-week for any date in history sounds like an impossible task for a normal person, but the algorithm is actually pretty simple to learn.

Although there are tutorials for this elsewhere online, and little quizzes available, there didn't seem to be anything well optimised with multiple practice modes etc.

I originally created a basic version of this just for myself, but a small group of friends and family found it interesting and gave suggestions such as the daily game.

Multi-lingual support is a bit rudimentary at the moment, it will only translate the month names and weekday names, not other text. If any translation mistakes are identified please let me know.

Other feedback is welcome too.

32 comments

jen729w2y ago

Dividing by 7 is also a cool trick and it’s way easier than this.

1/7th is 0.142857 recurring. All you need to remember is one-four-two-eight-five-seven. Say it over and over. It’s my favourite number.

The magic: to get 2/7, 3/7, any/7, just move the decimal point:

- 2/7: 0.285714285714…

- 3/7: 0.428571428571…

- 4/7: 0.571428571428… (I’m typing these out but ‘from memory’ but not really: only by knowing 1-4-2-8-5-7)

- 5/7 & 6/7: left as an exercise for the reader.

So then the trick is to play a bit of mental magic. Pretend like this is really hard and that every decimal point is stretching your mental capacity to its very edge.

31/7? That’s 4, with 3/7 left over. Say it out loud as you gaze in to the middle distance and do this difficult, difficult calculation in your head: “four point four two eight five seven … one four … two eight five … seven one four …” and so on.

A fun trick which is very occasionally practically useful.

hackernewds2y ago

that's really cool. but how does it help determine the day of week?

replwoacause2y ago

I don’t follow. How does this calculate the day of week?

account-52y ago

Not one part of this makes any sense to me.

midasuni2y ago

First question was “08/10/2023”

The majority of the world uses little endian, so the 8th day of the 10th month of the 2023rd year. 8th October is Sunday, as I know that because it’s the same day as 9th of May.

However experiments E has told me that much of the internet uses the minority American view of “middle endian”

Probably worth making it clear.

opan2y ago

Year-Month-Day (YYYY-MM-DD) is the international standard and ought to be used in any new projects.

benjoffeOP2y ago

Thanks, there is some attempt in the code to auto select US/Canada date format if the user's timezone is in the USA or Canada, but I didn't actually test it to see if it is activated correctly. Any US user's able to chime in if it does?

Even if it works, I'll probably take your suggestion add a tip below the date to clarify the date format for the user's first session.

tempestn2y ago

What do you consider the Canadian format? Most people I know prefer YYYY-MM-DD as it's non ambiguous. Anytime you see XX-XX-YYYY (or XX-XX-XX) it could just as easily be be little endian or weird American.

1 more reply

nonethewiser2y ago

It did not default for me.

Im in the US, Eastern TZ. Defaulted to English international.

Cool web app btw. I like the whole idea. Just a neat little self contained lesson.

pimlottc2y ago

Until you have time to test your localizations, you’re better off just using simple common format.

1 more reply

naniwaduni2y ago

Big-endian numerals in little-endian component order is still ixUn-endian! YMD or little-endian numerals, then we can talk :p

jbaber2y ago

I've found the Month's First Sunday method[1] (a mild simplification of the Doomsday Algorithm including the odd-plus-eleven innovation) easiest, fastest, and most practical. I'm not insisting on it, just great for me, and I wish it were better known.

[1]: https://firstsundaydoomsday.blogspot.com/2011/01/learn-by-ex...

hluska2y ago

My seven year old thinks I have superpowers now. Needless to say, I’m a big fan - it’s pretty amazing to go from learning to applying as quickly as I did. Great job!

keerthiko2y ago

Love it, why can I not choose YYYY/MM/DD as my date format though (tbh this should be default with manual selection to anything else!)

benjoffeOP2y ago

Thanks. I like your suggestion and I'll add a YYYY-MM-DD only option.

scotteh2y ago

This looks easier to memorise than another method: https://en.wikipedia.org/wiki/Doomsday_rule

Nice work on the site!

gcanyon2y ago

Some years ago I tried to simplify the Doomsday algorithm here: https://gcanyon.wordpress.com/2013/04/09/a-better-way-to-cal...

I don't think that's simpler than the algorithm at weekle, but in case anyone is curious.

robobro2y ago

How is it easier? I can do Doomsday in about 5 seconds for any year AD 1800 - 2200

rahimnathwani2y ago

I don't think I'll spend the time to memorize the part for the anchor days, but the observation that March 0, 4/4, 6/6, 8/8, 10/10, 12/12 all land on the same day will definitely come in handy.

1 more reply

entropy472y ago

Can you add more detail on the year table at the very bottom / how you're meant to use it? I can memorise it but I'm not sure how you're meant to use it after that.

benjoffeOP2y ago

Thanks, that's a good point that it does not clearly explain the use of that table. I will update the page soon to explain that the table cell represents the last 2 digits of a year, and the left column is the result of the year calculation for that year. The table has only 28 years in it since any year 28 or higher can be simplified by subtracting 28, 56 or 84.

I recommend only attempting to memorise the table after you are already able to calculate the year number using the normal algorithm.

quickthrower22y ago

What do you get for Sep 3 1752?

AdieuToLogic2y ago

> What do you get for Sep 3 1752?

An adjustment for clock drift[0]?

0 - https://www.historic-uk.com/HistoryUK/HistoryofBritain/Give-...

connordoner2y ago

I love the bit that sticks out on desktop with the QR code. Stellar work, Ben!

Retr0id2y ago

That's funny, I actually came here to complain about that. I find it visually distracting and there was no obvious way to dismiss it, so I had to use a content blocker.

benjoffeOP2y ago

Thanks, I'll add a dismiss button to that popout soon.

AdieuToLogic2y ago

> Mentally calculating the day-of-the-week for any date in history sounds like an impossible task for a normal person ...

  $ cal <month> <year>

And look :-)

onetokeoverthe2y ago

This was done ages ago.

j / k navigate · click thread line to collapse

32 comments

jen729w2y ago

Dividing by 7 is also a cool trick and it’s way easier than this.

1/7th is 0.142857 recurring. All you need to remember is one-four-two-eight-five-seven. Say it over and over. It’s my favourite number.

The magic: to get 2/7, 3/7, any/7, just move the decimal point:

- 2/7: 0.285714285714…

- 3/7: 0.428571428571…

- 4/7: 0.571428571428… (I’m typing these out but ‘from memory’ but not really: only by knowing 1-4-2-8-5-7)

- 5/7 & 6/7: left as an exercise for the reader.

So then the trick is to play a bit of mental magic. Pretend like this is really hard and that every decimal point is stretching your mental capacity to its very edge.

A fun trick which is very occasionally practically useful.

hackernewds2y ago

that's really cool. but how does it help determine the day of week?

replwoacause2y ago

I don’t follow. How does this calculate the day of week?

account-52y ago

Not one part of this makes any sense to me.

midasuni2y ago

First question was “08/10/2023”

The majority of the world uses little endian, so the 8th day of the 10th month of the 2023rd year. 8th October is Sunday, as I know that because it’s the same day as 9th of May.

However experiments E has told me that much of the internet uses the minority American view of “middle endian”

Probably worth making it clear.

opan2y ago

Year-Month-Day (YYYY-MM-DD) is the international standard and ought to be used in any new projects.

benjoffeOP2y ago

Even if it works, I'll probably take your suggestion add a tip below the date to clarify the date format for the user's first session.

tempestn2y ago

1 more reply

nonethewiser2y ago

It did not default for me.

Im in the US, Eastern TZ. Defaulted to English international.

Cool web app btw. I like the whole idea. Just a neat little self contained lesson.

pimlottc2y ago

Until you have time to test your localizations, you’re better off just using simple common format.

1 more reply

naniwaduni2y ago

Big-endian numerals in little-endian component order is still ixUn-endian! YMD or little-endian numerals, then we can talk :p

jbaber2y ago

[1]: https://firstsundaydoomsday.blogspot.com/2011/01/learn-by-ex...

hluska2y ago

My seven year old thinks I have superpowers now. Needless to say, I’m a big fan - it’s pretty amazing to go from learning to applying as quickly as I did. Great job!

keerthiko2y ago

Love it, why can I not choose YYYY/MM/DD as my date format though (tbh this should be default with manual selection to anything else!)

benjoffeOP2y ago

Thanks. I like your suggestion and I'll add a YYYY-MM-DD only option.

scotteh2y ago

This looks easier to memorise than another method: https://en.wikipedia.org/wiki/Doomsday_rule

Nice work on the site!

gcanyon2y ago

Some years ago I tried to simplify the Doomsday algorithm here: https://gcanyon.wordpress.com/2013/04/09/a-better-way-to-cal...

I don't think that's simpler than the algorithm at weekle, but in case anyone is curious.

robobro2y ago

How is it easier? I can do Doomsday in about 5 seconds for any year AD 1800 - 2200

rahimnathwani2y ago

I don't think I'll spend the time to memorize the part for the anchor days, but the observation that March 0, 4/4, 6/6, 8/8, 10/10, 12/12 all land on the same day will definitely come in handy.

1 more reply

entropy472y ago

Can you add more detail on the year table at the very bottom / how you're meant to use it? I can memorise it but I'm not sure how you're meant to use it after that.

benjoffeOP2y ago

I recommend only attempting to memorise the table after you are already able to calculate the year number using the normal algorithm.

quickthrower22y ago

What do you get for Sep 3 1752?

AdieuToLogic2y ago

> What do you get for Sep 3 1752?

An adjustment for clock drift[0]?

0 - https://www.historic-uk.com/HistoryUK/HistoryofBritain/Give-...

connordoner2y ago

I love the bit that sticks out on desktop with the QR code. Stellar work, Ben!

Retr0id2y ago

That's funny, I actually came here to complain about that. I find it visually distracting and there was no obvious way to dismiss it, so I had to use a content blocker.

benjoffeOP2y ago

Thanks, I'll add a dismiss button to that popout soon.

AdieuToLogic2y ago

> Mentally calculating the day-of-the-week for any date in history sounds like an impossible task for a normal person ...

  $ cal <month> <year>

And look :-)

onetokeoverthe2y ago

This was done ages ago.

j / k navigate · click thread line to collapse