2542: Daylight Calendar

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Daylight Calendar
Could be worse. In some towns north of here, it's already December, and the 21st will last for nearly a week.
Title text: Could be worse. In some towns north of here, it's already December, and the 21st will last for nearly a week.


At the time of this posting, the United States had ended Daylight Saving Time (DST) recently, on November 7, and returned to standard time. Daylight saving time is a practice of setting clocks ahead by 1 hour during warmer months to effectively 'borrow' some of the typically unused early morning light and pass it down to the late evening where more people can make use of it. In the United States, daylight saving time starts on the second Sunday in March and ends on the first Sunday in November.

A result of ending of daylight saving time is the sun setting earlier than people are used to, as people have become acclimatized to the shifted clocks — though it does mean an 'extra' hour of light has returned to the seasonally redarkening mornings. The start of the comic may be the start of a typical comment about how the sun seems to set earlier than usual in November; which it does anyway (north of the equator) but the clock-shift makes it even more obvious.

In this comic, however, Randall turns the normal talk about DST on its head by devising a calendar system where the dates "change" based on 12 hours of daylight. This causes shorter "days" in the summer months, which may get more than 12 hours of daylight in a "solar day" and longer "days" in the winter months which would have fewer hours of daylight in a "solar day". As mentioned in the title text, this change would be very pronounced near the poles, which may only get a few hours of daylight per 24 hours in the winter, but conversely may get 20 or more hours of daylight per 24 hours in the summer. Cueball says that he likes the new calendar system, as it gives him more "time" in the winter to complete work - if Cueball is given "3 days" to complete a task, each of those days could be longer than a typical 24 hours. However, this would be reversed in the summer, as each day would be shorter. Also, if this calendar system was in place, his boss could resolve this problem by just giving him 72 hours to complete his task instead of "3 days".

At temperate latitudes and above, as the calendar goes towards winter (for your hemisphere) the length of daylight per daily cycle shortens. Instead of having "long summer days" (i.e. periods of daylight) and shorter ones in the winter, but still the artificial pressures of a regulated 24-hour cycle to adhere to, the proposal seems to be that the date gets incremented whenever (and only when) twelve hours of daylight have passed.

In the summer, a day-count starting at sunrise could require a late-afternoon switch to 'tomorrow', which would in turn be switched earlier still the next day as it was already partly used up, with possibly two date-changes per astronomical day (early morning and mid-evening, for example). As winter draws on, not enough daylight will pass to guarantee a date-change in any single period. On the day of this comic's release - November 15, 2021 - Massachusetts, where Randall lives, gets ten hours and forty five minutes between civil twilights. It is possible that the last day-mark was late during the previous daylight cycle and the next one won't be until early in the following one.

Exactly how the time is marked is not fully explained. Starting each day-period at 00:00 would be easiest, but could be a psychological step too far. One possibility is to set a nominal 00:00 six hours before a day-change, in line with an 'idealized' twelve-hours-of-daylight day, but disregard hours 'belonging' to a prior daylight period. Then keep the clock running (throughout any intervening nights and into the next daylight as necessary) until the date clicks over and realigns as necessary. Clock times would not reach 23:59 for most of the summer, and could far exceed that in the winter. Megan's clock has reached 26:15, by this sunset, and may well be due to be far into the 30-hours range before more daylight and the moving on to the new date and reset time, if not beyond.

The title text relates to how days pass with respect to latitude. Depending on how exactly daylight is measured, we may have more "days" in a year than the usual 365, since refraction of light near the horizon means that the sun is visible slightly more than 50% of the year on average. This effect is strongest near the poles, since the sun spends more time near the horizon. In addition, due the Earth's elliptical orbit, more northerly parts of the Earth receive more sunlight than southerly parts. Combined, these effects mean that a year at the north pole is 381 "days", compared to 369 at the Equator.

This increase in the number of 'days' with latitude creates a situation where more northerly latitudes pass through the 'months' faster than lower latitudes, as beyond the Arctic circle, twelve hours of daylight would be accumulated upto twice per traditional day during the summer, while being effectively on hold for much of the other six months. This effect would be less pronounced outside the Arctic Circle but still create an acceleration in the passage of dates.

This creates the situation described in the title text, where towns north of Cueball's current location had increased daylight during the summer and hence accelerated through the calendar, and are currently in the '21st of December' at their latitude during winter, when periods of darkness are much longer, causing the time needed to change the date take nearly a week. This implies that they receive about 2 hours of sunlight every day in order to gain the required amount of sunlight in the timeframe given.


[Megan is looking down at her phone which she holds up in her hand, while Cueball stands next to her.]
Megan: Ugh, I hate November. It's 26:15 and the sun is setting again!
Megan: 3-day days are the worst.
Cueball: I like it. I know it's dark, but it's nice to have the extra time on deadlines.
[Caption below the panel:]
In our new calendar system, the date changes after every 12 hours of daylight, regardless of how long that takes.

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When did y'all in the US "fall back" your clocks? It has a look of being (askewedly) inspired by DST reversal, and I know you did one of the switches at a different typical weekend than us (UK BST>GMT was last weekend of October), but I thought it was 'first weekend of month-after-(the-month-that-it-is-our-last-weekend-of)'. You know, I could have just looked this up. 00:11, 16 November 2021 (UTC)

Second question, more easily expressed and less obviously answered, which sun-up/sun-down is this calculated by? Nautical, civil, etc? 00:11, 16 November 2021 (UTC)

I think at the equator, you get one day per day. At the pole you get two days per day in summer, then one six month long day.Template:Unsigned

Ah, I just mentioned that, in my edit. Though it depends upon how close to the pole as to how long you wouldn't get one full day for (and how the shifting boundaries align, possibly). I haven't worked out if those "further north" people are necessarily Arctic, or merely Canadian/northern-States even. I know that in the UK we're north enough to technically never get beyond civil twilight in the 'summer' months (the Sun isn't low enough below the horizon, as it passes below the northern rim, to be proper 'night') but we're still short of the actual Arctic Circle and true days-without-night/light, accordingly. I'm still not sure what edge-case is imagined. Perhaps intentionally left vague? 00:59, 16 November 2021 (UTC)

Is this supposed to be about whether it's cloudy? 00:17, 16 November 2021 (UTC)

I think not. If cloud cover were taken into account, the date would fail to ever sync up from place to place, and in heavily-overcast areas (*cough* Cleveland *cough) you'd only have a few days a year. Whereas, if cloud cover is irrelevant and a "day" is simply defined as the sun being above median horizon level (whether visible or not) for a total of 12 hours, then the date in each time zone will sync up once a year. Presumably you would arrange things so that this happens on New Year's Day. (If you don't arrange things and just switch to the calendar at some point in time, the date would sync up on the anniversary of the switch, assuming everyone switches at once.) Additionally, timezones would work roughly as they do now, if you travel due east or due west: driving west from Eastern time to Central, for example, would take you to a place where it's one hour earlier on the same date. Things would only get really weird if you go north or south at all, and even that would be sufficiently regular and formulaic that school children could be taught how to calculate it. --Tsadok

If I knew where to start (too many assumptions needed), I'd be tempted to make an "xkcd Calendar" that works like the xkcd Clock, but there are so many possible configurations (e.g. when is the 'epoch' of synchronisation? When do you count daylight from/to? Do you assume 6AM day-starts and work up from there?) before you then have to plug in your lat/lon to get your highly personalised datetime result that may well differ significatly even from someone a few miles away, when the time-boundaries involved have misaligned just enough and haven't shifted back together again (perhaps!) by the next epoch-point... 01:34, 16 November 2021 (UTC)

If you use the same epoch for everyone, the date and time would sync up within each timezone once a year, on the anniversary of the epoch. If everyone just naively starts at their then-current date/time on the switch date, then the switch date is the epoch. The ideal epoch and switch date for minimizing confusion would be midnight January 1st; this also has precedent (e.g., the Unix epoch is midnight January 1st 1970). This leaves the question of whether to start each day's time at 00:00:00, or start it at whatever you have to start it at to make noon happen at 12:00:00. The latter would mean starting the clock at negative times in the summer, positive in winter, and any given latitude's start-of-day times would average out pretty close to 00:06 over the course of a year. (The average wouldn't be _exactly_ 00:06 with infinite precision over a single year, because you're only averaging a finite number of days. But the average would asymptotically approach 00:06 over large numbers of years (unless the DST change being not-at-midnight or leap days being not-at-New-Year throws it off in a systematic way; but I am guessing this proposed calendar would replace and obviate DST; leap seconds *are* added at midnight, so they're ok; that leaves leap days as a potential monkey wrench if they aren't moved to align with the epoch), and it would be "close enough" for garden variety everyday purposes even after just one year.) --Tsadok
Ok, maybe go with this, start it at Unix timestamp 1609459200 (Midnight Jan 1st 2020, GMT), and you'd need to look up sunrise/sunset (0 degrees with respect to horizon) in the location (possibly round to the nearest degree of latitude for the closest conurbation, except when in the ocean) for the year. There may be a formula for that, or there may just be an API you can hit. Things wouldn't quite sync up, because the orbit isn't a unit number of sidereal days long. I'm wondering how bad the drift gets in 400 years. Erin Anne (talk) 17:14, 17 November 2021 (UTC)
Hi, I just wrote some python code that hopefully works to check an API for sunrise and sunset and then calculate from the start of the year: https://gist.github.com/ajlee2006/7c3e474d0e5d2e9b5daa7fd307abef2c 15:15, 8 December 2021 (UTC)
I like it, maybe add an explicit summary: "It is currently <datetime>, Day Number <day#>. Today will end at <day-end>, <x> seconds from now" Erin Anne (talk) 15:36, 8 December 2021 (UTC)

I know it is released close to daylight saving change. But has it actually anything to do with that? It is not mentioned at all, and only the darkness of November has any relation to the change. I'm not sure I would include it in the DST category... Randall has often made it clear that he dislikes DST but this new calendar is no guarantee they would not also include DST anyway. Hopefully we will stop with the DST in Europe from next year, so that we will not change back to summer time next spring! --Kynde (talk) 07:53, 16 November 2021 (UTC)

I thought it would have been closer to (US) changeover to be directly related. But perhaps Randall took a week to 'run some numbers' after being inspired, to get some (implausibly) plausible scenario to depict.
As for stopping putting the clocks forward... You could do that, but I'd suggest waiting a few years to see if people accept that before you also stop turning them back again in autumn, just in case... ;) 10:53, 16 November 2021 (UTC)
The conversation is clearly a parody of the kinds of conversations that occur regularly after the change to/from DST. I was at the eye doctor yesterday, and when she asked me about night driving I mentioned that this didn't really become an issue until the DST change, because now it's dark in the early evening. So these conversations do occur more than a week after the change. Barmar (talk) 17:18, 16 November 2021 (UTC)

Has anyone noticed the different style of Megan in this comic? I think that it may not be her… --Obscure xkcd reference (talk) 16:50, 17 November 2021 (UTC)

Three-Day Days[edit]

What I want to know is, what latitude do you have to be at, to get three-day days in November? --Tsadok

Anywhere north of the tropic, probably, if you count partial days (if a day starts before sunset, goes over anything even a smidgen less than 12 hours of daylight the next day, then completes itself soon after sunrise the day after).
But a bit of looking around suggests that today anywhere around 70°N has less than 4 hours of daylight so would easily need three full daylights in a row (or all-but) and possible unconsidered fractions of the neighbouring days depending on how the modified day-boundries land, which you may wish to ignore if you're strict). Wainwright, AK, would qualify, amongst other US settlements (most with native-names) and a number of Canadian ones, assuming you mean North from the US. There's Denmark (i.e. Greenland), Norway/Sweden/Finland and of course Russia with places too. Murmansk is slightly too far south today, I think, but I haven't checked for later dates in November and I'm sure it'll get included before too much longer.
Looking for a current six-hour daylight (i.e. any daylight not included in the period the day trips over upon is exactly made up for at the beginning of the third day touched before the new-calendar day ends) suggests reaching ~65°N will suffice, which adds a fair few other places 'up north' that would apply to right now. 21:15, 16 November 2021 (UTC)

Of course, before mechanical clocks, all days were 12 hours long. It is just that the length of the hour varied, as it was always one twelfth of the day, or night. Arachrah (talk) 18:04, 17 November 2021 (UTC)