Doctor Strangeclock
Or How I Learned To Stop Worrying And Love Springing Forward
This weekend is the “spring forward” time change in the US, from Standard Time to Daylight Saving Time, which often creates a flood of thinkpieces. (This seems to be somewhat restrained this year by all of the… [expansive gesture] stuff going on at the moment.) As someone who wrote a whole book on timekeeping (available wherever you buy books, and many places you don’t) I have occasionally contributed to this flood; this year, I haven’t been asked to write anything, but it feels strange to sit it out completely, so I’ll offer something a little flippant here.
The original justification for the twice-a-year clock shift is energy: it started in Europe during WWI as a way to conserve fuel. By moving clocks forward in the springtime, there was less need for artificial light in the summer months, and fuel that otherwise would’ve gone to making light could be used for military purposes instead. This got picked up more widely during the war, and was formally implemented by the US Congress in 1918 (in a law that also codified the boundaries of the time zones). It continued somewhat intermittently between WWI and WWII, then was widely adopted again in the 1940’s, and stuck around after until being codified in the Uniform Time Act of 1966. The exact dates of the time change and which places participate have shifted a bit over the years, but the system has stayed more or less the same since then.
Through all of this, the justification through appeal to energy savings has remained pretty consistent. Even among the non-observant states, the argument turns on energy. Hawaii has never participated in DST, the logic being that since they’re much closer to the equator than the rest of the US, the change in length of day isn’t as extreme (a swing from 13.5 hours of daylight in summer to 11 in winter, compared to 16 and 10 in Manhattan), so any savings they might see isn’t worth the bother. Arizona also skips out on DST, again citing energy use: the modern argument is that shortening the summer days reduces the number of hours that people are awake and running their air conditioning.
The Arizona example points to a flaw in the energy argument, though, which is that nowadays air conditioning is basically ubiquitous in the US in the summer, and cooling air at scale takes way more energy than generating light. I haven’t seen anyone make a good run at the numbers, but it wouldn’t surprise me to learn that we’d save energy on net by going to year-round Standard time.
But I think the time change ritual is ultimately worth keeping for reasons that are more philosophical than practical. The twice-yearly changing of the clocks is worthwhile because it reminds us that “What time is it?” is not a well-formed question with a universal answer, but a matter of convention. As tempting as it is to think of clock time as some kind of objective and absolute quantity, at the end of the day (or in the middle of the day, or the middle of the night) there’s an arbitrary choice being made at the very core of the system. We should embrace that.
This is most clear when looking seriously at one of the more common arguments advanced for keeping Standard Time year-round. When current events are less eventful, the time change inevitably brings a couple of articles by or quoting sleep doctors talking about natural cycles and how we’re adapted to the cycle of the Sun. Everybody would sleep better and generally be healthier, they say, if we stuck to Standard Time because that more closely matches nature.
The problem with that is that this match is only really good if you happen to live near the center of a time zone. If you’re near one of the edges, though, you’re already well out of synch with any kind of natural cycle. The Sun reaches its highest point in the sky at 12:55pm in Indianapolis, IN (longitude 86.16 W) today, while in Lubec, Maine it got there at 11:39 am. The “natural cycle” times for these cities are an hour and a quarter apart, but we have chosen to assign them both to the same time zone, because it’s just more convenient if we do it that way.
The natural cycle argument isn’t really an argument for Standard Time so much as it is an argument for local time— building your day around sunrise, sunset, and solar noon, which vary substantially as you move from place to place. That’s the “natural” condition for human lives, but we don’t do that any more because modern transportation and telecommunications make it super awkward for the time of day to shift continuously as you move. We make the arbitrary choice to set clocks in eastern Maine and western Indiana to the same time because everybody would go nuts if we let every individual town have its own time.
This also gets at a key point regarding policy and expertise, one that I tend to associate with Josh Barro, because I liked the way he formulated it the first time I heard it. Expertise can tell you what will happen in a particular domain if you implement a particular policy, but it can’t tell you what you want to happen. That’s a question of what you value, and for any situation complicated enough to have an interesting conversation about, that will necessarily involve trading different issues off against each other.
It might very well be true that, from a narrow medical standpoint, everybody would sleep better if we adjusted our schedules to be in synch with the Sun. We don’t do that, though, because it’s easier to coordinate activities over long distances if we standardize time across broad geographic reasons. And we have decided as a society that we value the ability to collectively get shit done more than we value local sleep schedules.
Similarly, the actual benefit of turning the clocks back in the fall and forward in the spring has little to do with patterns of energy usage, but is much more psychological: collectively, we enjoy the long summer evenings that Daylight Saving Time affords us. In much of the US, it’s possible to come home from work in the summer, have dinner, and spend another hour or two enjoying outdoor activities before it gets dark. Over the last hundred-odd years, that’s become a cherished part of summertime— literally, the summer time that gives us an extra hour of nice weather.
So, why not dispense with the changes and stick with the time zones as they are under DST year-round? Because while people like long summer evenings, they hate dark winter mornings even more. In New York City under Standard Time, the Sun rises a bit after 7am in December; in a year-round DST scheme, it wouldn’t come up until after 8am. In Indianapolis, the December sunrise would be after 9am.
I’m already getting up before the sun for the better part of six months a year— my alarm is set for 5:15 am, and I’m usually already up when it sounds— so you can believe me when I say that December mornings are not a ton of fun. The worst part by far of this weekend’s time change, for me, will be that it means dropping back into walking the dog in the dark for a week or two, and I don’t like to think about what another hours of December darkness would feel like.
But people who wake up two hours after I do complain now about mornings being dark and depressing in winter. You really don’t want to know what they would be like with the sun rising after 8am.
In fact, this was already tried, back in the depths of the energy crisis of the 1970’s. In December 1973, Congress voted to switch to year-round Daylight Saving Time for two years, as an experiment, with the clocks shifting forward on January 6, 1974. People hated getting up in the dark so much that the two-year trial run didn’t make it a full year— the change, and the clocks, were rolled back in October.
As awkward as the changing of clocks is, the current system gets us two things that we enjoy: longer summer evenings, and brighter winter mornings. Those benefits are balanced against the cost of losing an hour of sleep one spring weekend (offset a bit by gaining an hour in the fall…)1, in the same way that the coordination benefits of wide geographic time zones are balanced against the cost of sub-optimal circadian rhythms for people in Maine and Indiana.
As a physics professor, I also find some pedagogical benefit in the twice-a-year clock switching. Admittedly, it doesn’t do wonders for class attendance in March, but when it comes to teaching the physics of relativity, it’s a useful counter to the kind of instinctive Aristotelianism students come to college with. A lot of people recoil from the idea of clocks in different places showing different times, thinking of time as a universal absolute. The combination of time zones and DST clock changes helps make the point (or, rather, remind students of a thing they already know but don’t fully appreciate) that time is to a large degree a matter of convention. The proper answer to “What time is it?” at some level is “Whatever time we agree to say that it is,” and from there we have agreed-upon rules to predict physical phenomena.
(A bit of an aside: I often assign students to read Henri Poincaré’s The Measure of Time when teaching relativity2, because it does a really nice job of laying out the conventional nature of time. Those 19th century polymath guys were pretty sharp…)
So, there’s the case for why I’m good with changing clocks twice a year: it’s a system that gets us several months of things that are good (late summer sunsets, early winter sunrises), in exchange for a couple of brief periods of badness. Plus, it makes my job a tiny bit easier in a couple of courses by reminding people that time isn’t actually something you can read off a giant universal clock, but a more fluid thing. And that idea opens a huge range of fascinating physics…
That’s Spring 2025’s version of “Why I don’t support Daylight Saving Time reform;” I could probably write something like this twice a year in perpetuity. If you want to see whether I do, here’s a button:
And if you have any novel arguments for changing the system (“Switching to year-round Daylight Saving Time this year will shorten by one hour the time that Donald Trump is President of the United States” is pretty compelling…), the comments will be open:
This is the place to plug my flippant alternative scheme: we do the “fall back” change as usual, because everybody could use an extra hour of sleep, but eliminate the “spring forward” change by replacing it with a five-minute shift forward every weekend for twelve weeks starting at the solstice. That gets us back to DST in March, without the shock of losing one weekend hour, which is the thing that people who aren’t used to walking dogs in the pre-dawn hours actually hate most about the current system.
Which, credit where due, I learned about from Peter Galison’s Einstein's Clocks, Poincaré's Maps, which I also recommend.
My brother argues that we'd get a lot more buy-in for the "spring forward" part of this if the springing forward happened at 4pm on a Friday... because then all of a sudden it'd be time to head home or hit happy hour.
In the fall, my dad used to keep the clocks on daylight time until about 3 or 4 in the afternoon, then fell back to, in a sense, replay an extra hour of Sunday afternoon. But with most of the clocks around me now automatically adjusting it's harder pull that one off.
Five minutes before reading this, I was reading a thread on physicsforums.com on one way speed of light and how its a matter of time conventions. I was thinking it would be weird if your post went in that direction.