The march of time is inexorable. None among us can escape its ravages nor its whims. Time’s also messy, and while we’d love for it to line up cleanly with the physical phenomena we use to keep track of it, that’s frustratingly rarely the case. Plenty of efforts have been made to deal with that over the years, and now the latest push is coming from an unlikely source.
Most of us are familiar with leap years: when one day is inserted into the calendar in February to account for our trip around the sun taking approximately 365.25 days, rather than an even 365. But it’s not just our years that aren’t easily expressed in whole numbers, and the length of our days isn’t exactly 24 hours, either. To account for this discrepancy, the
Time Variance Authority International Earth Rotation and Reference Systems Service (IERS) mandates a leap second be added to our clocks as the need arises. Unless, of course, Mark Zuckerberg has anything to say about it.
The reason we’re talking about the minutiae of time keeping is because Meta has penned a blog post advocating for the abolition of the leap second. Although its argument has some merit, in typical Facebook fashion, it does a poor job of making it, dismissing the leap second as it “mainly benefits scientists and astronomers,” when in fact, astronomers were some of the first to argue against it.
So why does Meta (and admittedly, other industry big shots like Google, Microsoft, and Amazon) want to do away with leap seconds? Why do government agencies like the National Institute of Standards and Technology agree? To understand, let’s start with the big picture and work our way down to the details.
What is a “second,” anyway?
The earth throttles around the sun at something like 67,000 mph, and one complete rotation is how we define our year. While the whole We whirl about the poles at 1,000 mph (at the equator) and from one complete rotation we define our days. Around the turn of the first millennium, the Muslim scholar Al-Biruni split that day into 24 parts, which he further subdivided sexagesimally (60 parts) into minutes and seconds. The second was therefore ¹/₈₆,₄₀₀ of a day.
During the 19th century the need arose for a standard second and scientists realized that the length of a day wasn’t constant — so, defining a second as a fraction of a day meant that it didn’t have a fixed value. One temporary solution was to define the second as a fraction of a year, but ultimately it was decided that the second should be separated entirely from the movements of heavenly bodies. In 1967, the second was redefined to conform to the atomic properties of cesium (the details of which are beyond the scope of this article) … and here our problems begin.
Isidor Rabi and the first cesium atomic clock, NBS-1
Before atomic clocks came onto the scene, the world had already agreed upon Universal Time (UT) as the international standard of time. It was based on the Prime Meridian and the position of the stars. The standard that would become International Atomic Time (TAI) began in 1958 and Coordinated Universal Time (UTC) was adopted in 1960. Even though UTC was based off of TAI, in order to maintain a loose parity with UT, the length of the UTC year (the rate at which the clock ticked) had to be tweaked each year, so that the two standards would be in agreement.
The UTC second wasn’t adjusted to match the TAI second until 1972, by which point it was 10 seconds ahead of UT. In order for UTC to account for the drift between TAI time and UT going forward, it was decided that leap seconds would be added as needed to keep UTC from drifting too far from the solar-based UT.
I’m bored. What does this have to do with computers?
Computers rely on clocks and timers to do all kinds of things. More and more, they also talk to each other. And to make sure everyone’s on the same page, computers rely on the Network Time Protocol (NTP) to ensure that their clocks are synchronized with millisecond precision. This synchronization is vital for services like stock transactions where time of purchase is critical information. Computers working in concert also often need to execute a series of actions in a specific sequence, and they rely on sharing access to the same clock data to make sure that happens.
Leap seconds become a problem because NTP isn’t a monolith — there are around 3,000 public NTP servers, and they don’t always work in unison. For computers that operate on the scale of milliseconds, the difference of a second is massive and can wreak havoc with systems that weren’t designed to handle it.
After the last leap second was added on New Year’s Eve 2016, Cloudflare suffered a partial outage due to the extra second causing a negative value to be passed to a critical function. In the summer of 2012, Reddit was knocked offline for an hour due to the extra second triggering all of its internal timers at the same time, overloading its servers. And in 2015, the New York Stock Exchange delayed its opening in anticipation of potential leap second impacts.
Although the digital repercussions of an extra second in our day tend to matter less every year, that can still be a headache for everyone involved, especially since leap seconds aren’t inserted in a regular interval and they’re only announced six months before they happen. We’ve also only had five of them since 1999, so plenty of online services were developed without having to account for the leap second. And even when it was accounted for, there have only been five opportunities to evaluate the consequences.
What to do with the leap second
The problem with UTC as a single universal standard is that it tries to marry the accuracy of atomic time with the mutability of solar time. Computers require rigid regularity and thus benefit from the atomic precision provided by TAI. Humans, on the other hand, are ruled by our circadian rhythms and prefer something like UT, which is based on the sun.
A GPS Block III satellite
Meta’s proposed solution to this conundrum is the wholesale abolition of leap seconds. UTC would still be the international standard of time, but it would be detached from the sun, drifting by a second every 800 days or so. That means sunset will come one minute earlier in 2155, and a full hour earlier in about 8,000 years. Even though Meta’s plan is essentially kicking the can down the road a few thousand years, the loss of the leap second wouldn’t be felt for a long time.
Another alternative is to use two time standards. Computers, which require immutable time, could remain on an unchanging UTC standard, as Meta proposes. And when it comes to solar time, the leap second could continue to be used by us meat bags without affecting critical digital infrastructure. This wouldn’t be too difficult to implement since GPS satellites already use their own time standard (which doesn’t have leap seconds) and computers have no problem interacting with them.
There have been active talks about removing leap seconds since 2005 but so far nothing has been decided. Any decision would have to begin at the International Telecommunications Union, which is responsible for defining UTC. The last time it was seriously discussed was in 2015, but any decision was deferred until 2023 — which is coming up on us soon. Given that the US, China, and tech giants like Meta, Google, Microsoft, and Amazon are all in favor of eliminating the leap second, be on the lookout for a change to UTC in the future, but don’t expect it to affect your daily life… unless Reddit goes down again.