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Lewis Carroll – computing the day of the week for any given date (1887)

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Re: Lewis Carroll – computing the day of the week for any given date (1887)

#3

Post by ColinWright »

This is very similar to the method I use and, after conversation with him, the method Art Benjamin uses. It's trivial to do in 10 to 15 seconds, though it requires practice, a very small amount of memorisation[0], and a small amount of mental arithmetic. I use it regularly, partly to keep in practice, and partly because once you have the skill you find it's surprisingly useful.

JH Conway used a different technique[1] which I have swutch to when computing days in the current year. It's quicker and easier, but I find that it's harder to compute "The Doomsday" for other years (it's Thursday this year), so I revert to my standard method[2].

Example: Today is 2024/05/24

    Years since 2012 is 12
    Leap years is 12/4 = 3
    Magic month number = 2
    Date is 24
    Add mod 7 is (12+3+2+24) = (-2)+3+2+3 = 6 = Friday
[0] A "magic number" for each month. There are mnemonics, and they can be computed from first principles if needed. It's also easy to remember a few and then compute the others.

  144 : Jan, Feb, Mar
  025 : Apr, May, Jun
  036 : Jul, Aug, Sep
  146 : Oct, Nov, Dec
[1] https://en.wikipedia.org/wiki/Doomsday_rule

[2] All values computed mod 7. Take years since 1900. Add number of leap years since 1900 (so (YYYY-1900)/4 rounded down). Add the magic number for the month. Add the day of the month. Then Sunday=1, Wednesday=4, etc.

Because the calendar repeats every 28 years, for more recent dates you can start with 2012 instead of 1900.

Re: Lewis Carroll – computing the day of the week for any given date (1887)

#4

Post by throwaway211 »

Reading the article, I wandered into the difference of Old Style and New Style dates and what happened in 1752.

From [1]: By the 18th century, the English legal year – used for legal, financial and other civil purposes – had for centuries begun on 25 March, or Lady Day.[13][i] Thus, for example, 24 March 1707 was immediately followed by 25 March 1708, while the day following 31 December 1708 was 1 January 1708, with 1709 still nearly three months away.

Thank goodness that happened.

[1] https://en.m.wikipedia.org/wiki/Calendar_(New_Style)_Act_175...

Re: Lewis Carroll – computing the day of the week for any given date (1887)

#5

Post by [deleted] »

ColinWright wrote: post #3

This is very similar to the method I use and, after conversation with him, the method Art Benjamin uses. It's trivial to do in 10 to 15 seconds, though it requires practice, a very small amount of memorisation[0], and a small amount of mental arithmetic. I use it regularly, partly to keep in practice, and partly because once you have the skill you find it's surprisingly useful. JH Conway used a different technique[1]…

[deleted]

Re: Lewis Carroll – computing the day of the week for any given date (1887)

#6

Post by jihadjihad »

ColinWright wrote: post #3

This is very similar to the method I use and, after conversation with him, the method Art Benjamin uses. It's trivial to do in 10 to 15 seconds, though it requires practice, a very small amount of memorisation[0], and a small amount of mental arithmetic. I use it regularly, partly to keep in practice, and partly because once you have the skill you find it's surprisingly useful. JH Conway used a different technique[1]…

> which I have swutch to

That's a past participle I've never seent before

Re: Lewis Carroll – computing the day of the week for any given date (1887)

#7

Post by jgrahamc »

ColinWright wrote: post #3

This is very similar to the method I use and, after conversation with him, the method Art Benjamin uses. It's trivial to do in 10 to 15 seconds, though it requires practice, a very small amount of memorisation[0], and a small amount of mental arithmetic. I use it regularly, partly to keep in practice, and partly because once you have the skill you find it's surprisingly useful. JH Conway used a different technique[1]…

I assume by

  144 : Jan, Feb, Mar
  025 : Apr, May, Jun
  036 : Jul, Aug, Sep
  146 : Oct, Nov, Dec
You mean: Jan = 1, Feb = 4, Mar = 4, etc.

Re: Lewis Carroll – computing the day of the week for any given date (1887)

#8

Post by chongli »

jgrahamc wrote: post #7
ColinWright wrote: post #3

This is very similar to the method I use and, after conversation with him, the method Art Benjamin uses. It's trivial to do in 10 to 15 seconds, though it requires practice, a very small amount of memorisation[0], and a small amount of mental arithmetic. I use it regularly, partly to keep in practice, and partly because once you have the skill you find it's surprisingly useful. JH Conway used a different technique[1]…

I assume by 144 : Jan, Feb, Mar 025 : Apr, May, Jun 036 : Jul, Aug, Sep 146 : Oct, Nov, Dec You mean: Jan = 1, Feb = 4, Mar = 4, etc.

Yeah, since they’re all mod 7.

Re: Lewis Carroll – computing the day of the week for any given date (1887)

#10

Post by reedf1 »

ColinWright wrote: post #3

This is very similar to the method I use and, after conversation with him, the method Art Benjamin uses. It's trivial to do in 10 to 15 seconds, though it requires practice, a very small amount of memorisation[0], and a small amount of mental arithmetic. I use it regularly, partly to keep in practice, and partly because once you have the skill you find it's surprisingly useful. JH Conway used a different technique[1]…

I am struggling to understand the advantages over the Lewis Carroll method, which I find very simple and quick. It seems to add quite a lot of complexity.
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