Talk:1935: 2018

Explain xkcd: It's 'cause you're dumb.
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This is easy! Don't factor it - just multiply by 25 and if that ends in two zeros, but not four zeros then it's a leap year, at least most of the time.....17:25, 29 December 2017 (UTC) (talk) (please sign your comments with ~~~~)

This is easy! Don’t factor it - just convert it into a binary and look at the 2 least significant bits. If they are 00 the number is multiple of four. — 17:37, 29 December 2017 (UTC)

This is easy! Don't factor it - just subtract 4 repeatedly. If you end up at 0, it's divisible. If you end up at 1, 2, or 3, it's not. -- 17:55, 29 December 2017 (UTC) (talk) (please sign your comments with ~~~~)

This is easy! Sums of numbers that have 4 as a factor are all divisible by four. (I'll leave the proof of that as an exercise for the reader, but it's really trivial, though possibly non-intuitive.) This means that one can take a number apart and check the individual pieces. Now, any number that's a multiple of 100 is divisible by four (10 * 10 = 5² * 2²,) so one can essentially cut away the higher digits of a number, as they do not influence its divisibility with regard to 4. Now look at the first of the remaining digits. If that's odd, add 2 to the last digit. If the last digit is now divisible by four, the original number is divisble by four. Tibfulv (talk) 00:38, 30 December 2017 (UTC)

The calculation of Christmas is trivial[citation needed] it's December 25th. Where as the calculation of Easter is complex ([1]). 18:03, 29 December 2017 (UTC)

Calulsting the date of Christmas is actually non-trivial. It depends on your location. For example if you are in the US it's in December. If you are in Russia it's in January. If you are in Ukraine it's sortof both but not really. And if you are in Crimea, well, see one of the 2 previous sentences. -- 15:22, 30 December 2017 (UTC)

Actually, it's December 25 on the old Julian calendar in Russia. It's just that Russia uses the Julian calendar for liturgical purposes and the Gregorian calendar for secular purposes. It's a bit schizophrenic. Billjefferys (talk) 19:40, 30 December 2017 (UTC)
I've moved some content to the trivia section and added this Julian calendar issue for the Eastern church.--Dgbrt (talk) 19:42, 30 December 2017 (UTC)
Title text explanation mis-read

Explanation of title text is incorrect: "The title text refers to calculating the date of Christmas; again, this is a trivial exercise, because Christmas is always December 25." Title text states 'day of Christmas', not 'date...'. The day changes each year and so does require calculation. (talk) (please sign your comments with ~~~~)

Oops, my bad. Fixed. FlyingPiMonster (talk) 18:08, 29 December 2017 (UTC)

  • I think you have it backwards. The title text is a reference to calculating the day (as in "date", not "day of week") of Easter. This is a non-trivial calculation (though one that modern computers can perform easily). On the other hand, the Christmas day is fixed. (There's no reason to believe that the joke was anything else.) - Mike Rosoft (talk) 19:13, 29 December 2017 (UTC)

I don't know who wrote the explanation, but... Are they having a bad day? 18:44, 29 December 2017 (UTC)

That was vandalism. I did a revert. --Dgbrt (talk) 19:06, 29 December 2017 (UTC)
Ah, no, I was asking because the explanation sounds so angry. 22:48, 29 December 2017 (UTC)
Also, Megan understands that checking if a number divisible by 2 is easy 19:32, 29 December 2017 (UTC)
Theory for possible explanation

Didn't want to edit this in because I'm not sure- but the motivation for this uncharacteristic lack of mathematical rigor could have to do with the current trend of people being dismissive of science being able to predict things. Something that seems pretty obvious is made to look like a chance event that nobody can really predict ahead of time. -- Sirpent (talk) (please sign your comments with ~~~~)

This is easy! Don't factor it - just subtract 2000. Is 18 divisible by 4? If so, you're an idiot. (talk) (please sign your comments with ~~~~)
The nonsense does look to me like a political discussion where one person uses "alternative facts". But in real life people get leap years "amusingly" wrong. Computer system designers for instance... one software tool I used passed into the year 2000 working correctly, but then it broke 2 months later because it thought 2000 wasn't a Gregorian calendar leap year, I guess because every 4th year is but every 100th year isn't. Every 400th year is, but, if the programmer just stopped at "every 4th is a leap year" then they'd have been fine until 2100. Robert Carnegie [email protected] 22:06, 29 December 2017 (UTC)

The joke in this might be that it might take some time to brute-force the prime factorisation of 2018 with a calculator as it’s 2*1009. Same holds true for 2017 which is prime. Therefore on might come to the conclusion that factorisation is hard already at this scale. (flx) 22:24, 29 December 2017 (UTC)

Odd/even is another joke

Cueball: No, it's definitely not. Leap years are divisible by 4. Megan: Right, and for odd numbers, that's easy. Megan: But 2018 is even.

She can see that finding out if a number is divisible by 2 is easy, but for dividing by 4 it's a "50/50 chance", and really hard to calculate. IMHO the best joke in the comic but missing from the explanation. 23:59, 29 December 2017 (UTC)

I think it's interesting that 2018 only has two factors, 2 and 1009. Maybe a trivia? 17:40, 30 December 2017 (UTC)

I think there should be a mention of leap year rules in general, since they are nontrivial (divisble by 4, except not multiples of 100, except yes to multiples of 400)? 18:43, 30 December 2017 (UTC)

I think the joke in the title text is a play on the old joke that even though we know every year when Christmas falls, and every year we always say that we are going to begin saving or shopping in the months preceding Christmas we always get to December and are "surprised" that Christmas happens to be in December. Effectively Randall is suggesting that the reason we are surprised Christmas is in December is not due to forgetting but rather that we are "calculating" when the day is. Also related are Jokes about American Tax day (April 15th) or pretty much anything to do with procrastination. 01:06, 31 December 2017 (UTC) Sam