Title text: We should really start calculating it earlier, but until the end of December we're always too busy trying to figure out which day Christmas will fall on.
In this, the first of two New Year comics in a row, Megan wonders whether 2018 will be a leap year. Cueball thinks 2018 will not be a leap year, and Megan responds that she "doubts anyone knows at this point." This appears to be a jab at people who suggest that anything they don't know is generally unknown. As Cueball says, leap years occur every four years (though there are a few exceptions - a year divisible by 100 is not a leap year, unless it is also divisible by 400), adding an extra day to account for the fact that Earth takes a bit longer than 365 days to orbit the Sun. Therefore, most years that are a multiple of four are leap years. As Megan says, this is easy for odd-numbered years, since no odd numbers are divisible by four. However, for even-numbered years, it isn't quite as simple. (Though, since the number 2,000 is evenly divisible by 4, the problem can be reduced to the much simpler question of whether the number 18 is divisible by 4.)
The last panel expresses a misunderstanding of modern public-key cryptography, which relies on the fact that it is difficult to factorize large numbers. Megan is applying this concept to the year, claiming that it is hard to determine whether or not 2,018 is a multiple of four and hence is a leap year. In reality, factorization is not needed here, since we already know the factor in question, which is four. Megan states that, if it were possible to factor large numbers with a calculator, modern cryptography would collapse. While true, it is true only for truly large numbers (hundreds of digits), and no factorization is needed in this case.
At the end of the strip, Megan hopes the answer can be brute-forced by February. Brute force is a method of breaking cryptography by trying every possible option until one works. This is misdirection upon misdirection, in that, even if we needed to factorize 2,018 (which we don't), the simplest brute-forcing algorithm would need to try only 14 numbers -- each prime from 2 to 43 (the square root of 2,018 is closest to 44). In cryptography, the algorithms use numbers much, much bigger than 2,018 -- on the order of hundreds or even thousands of digits.
The title text refers to calculating which day Christmas will fall on. As Christmas always lands on December 25 by definition, the day of the week varies from year to year, though it's always the 359th or, in leap years, the 360th day of the year. Still, determining which day of the week December 25 lands on is not a difficult problem to solve, requiring only a few mathematical operations to compute. Alternatively, this might be an oblique reference to Easter, the date of which jumps from year to year according to a multi-layered algorithm that most people don't know. The changing date of Easter was recently included in 1930: Calendar Facts.
- [Megan is walking.]
- Megan: I wonder if 2018 will be a leap year.
- [Now it turns out that Cueball walks behind Megan.]
- Cueball: ...it won't be, right?
- Megan: I doubt anyone knows at this point.
- [Same scene in a frame-less panel.]
- 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.
- Megan: 50/50 chance.
- [Zoomed-out view with both walking in silhouette on a dark slightly curved ground.]
- Cueball: I can settle this with a calculator.
- Megan: No way. If it were easy to factor large numbers like that, modern cryptography would collapse.
- Cueball: I see.
- Megan: I just hope we manage to brute-force it by February.
- Released on Friday, December 29, this was the last comic of 2017. The next comic, 1936: Desert Golfing, was released soon after midnight (in Randall's time zone) on New Year's Day 2018.
- Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 were not leap years, but the years 1600 and 2000 were.
- Since 100 is divisible by 4, only the last two digits of a number are needed to determine if that number is divisible by 4. So to determine if 2018 is divisible by 4, we only need to check whether 18 is divisible by 4, which is easy.
- 2018 is not divisible by 4, so the year is not a leap year. 2016 and 2020 are leap years. This is true for both the Gregorian and the Julian calendar. A year is roughly 365.2422 days long.
- Eastern Christian Churches celebrate Christmas also on December 25 but on the older Julian calendar, which currently corresponds to January 7 on the Gregorian calendar.
- This is the third year in a row with New Year's comics with only the year used as the title; before that there were two more comics with such titles, but those two (and thus the first three) were only released in the even years: 998: 2012 in 2012, 1311: 2014 in 2014, 1624: 2016 in 2016 and 1779: 2017 in 2017.
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