It’s week four and this week we’re talking about the inconsistencies, the problems, the strange things about time and how we measure it. If week three was mostly about software and computer systems, week four is more about the properties or the features or the details of recording time and our time systems.

The first crack in our concept of time is the quite irregular leap second.

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UTC works?

How can a minute ever have 61 seconds? If you start at zero, you got to end at 59. You start at zero and you end at 60, now you have 61 seconds. That doesn’t make sense.

If you have ever seen :60Z in a log file, that is the leap second. That minute had sixty-one seconds in it.

It exists because we are trying to do the impossible: keep two completely different definitions of the “second” in alignment, forever. There are, it turns out, two definitions.

But wait, it gets worse.

There are three time scales running in the background of our civilization right now:

• TAI, or International Atomic Time. The average of about 400 atomic clocks at standards labs around the world, all ticking off the cesium hyperfine transition we covered on Day 8. TAI is uniform. Every second is the same length as every other second. TAI does not care about the Earth.
• UT1, or Universal Time 1. Defined by the Earth’s actual rotation, measured by tracking distant quasars with radio telescopes. UT1 is wobbly. The Earth speeds up and slows down by milliseconds per day, mostly because of tidal friction (slowing it down) and core-mantle coupling (anyone’s guess on any given decade).
• UTC, or Coordinated Universal Time. The civil time on your phone. UTC is TAI minus an integer number of leap seconds, kept within 0.9 seconds of UT1.

It’s time to stop pretending like the current version of UTC isn’t a compromise. It certainly is not the best we could come up with. It’s just what everyone could agree on.

The first of its problems is the dang leap second.

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So how did we get to this mess?

The IERS, the International Earth Rotation and Reference Systems Service in Paris, watches the gap between UT1 and UTC. They announce the leap second six months in advance in an actual notice called Bulletin C. They also have their own weekly and monthly newsletters called Bulletin A and Bulletin B. I don’t know what is going on at IERS and I’m sorry to anyone working there, but this leap second thing is kinda crazy.

When the leap happens, the clock reads:

23:59:58
23:59:59
23:59:60   ← this is the leap second
00:00:00

That :60 is the part that breaks software. Most date/time libraries do not believe :60 is a real value. POSIX, the standard governing Unix systems, explicitly defines Unix time to pretend leap seconds don’t exist.

Since the system was introduced in 1972, 27 leap seconds have been inserted, although none since 2016. Also, there has never been a negative leap second. We’ve only ever needed to slow UTC down to match the Earth.

But, that may be about to change. More on that tomorrow.

Because I can’t help myself.

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Here’s more stuff about… Computers.

There are three ways a computer can handle the leap second arriving.

1. Step. At midnight, the clock just jumps back one second. From the OS’s perspective, time briefly moves backward. Anything assuming time is monotonic, meaning it only goes forward, sees its assumption violated and may explode.
2. Stall. Hold 23:59:59 for two seconds. Time doesn’t go backward, but two events get the same timestamp. Anything depending on timestamp uniqueness gets confused.
3. Smear. Spread the extra second over a long window (Google originally used 20 hours centered on the leap, then standardized at 24 hours) by ticking slightly slow for the whole period. No :60 ever appears. No backward step. Just a clock that runs 1.0000116× slow for a day.

Google introduced smearing in 2008. By the late 2010s most cloud providers (Amazon, Microsoft, Facebook) had adopted some flavor. It is now the de facto practice.

Before smearing, leap seconds were MORE dangerous.

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A second is a second not two

A leap second is like when Pluto was a Planet. It has to be a singular definition. A known amount. A standard. Software written at some of the most capable engineering organizations still took down important infrastructure, internet infrastructure. Clearly it should not be this hard to define a unit of time.

But anyways, onto the next post.

Tomorrow will cover the historic 2022 vote to abolish the leap second, the fight that produced it, and what we all agreed to.

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Sources

• International Earth Rotation and Reference Systems Service (IERS) — The body responsible for monitoring Earth’s rotation and issuing Bulletin C to announce leap seconds.
• A global timekeeping problem postponed by global warming — Nature (2024). The Duncan Agnew paper detailing how melting polar ice has counteracted the Earth’s acceleration, delaying the unprecedented “negative leap second” until roughly 2029.
• POSIX.1-2017 Base Definitions: Seconds Since the Epoch — The Open Group. The formal specification demonstrating that Unix time legally ignores leap seconds.
• Time, Technology and Leaping Seconds — Google’s original 2011 blog post introducing the concept of the “leap smear.”
• The Leap Second Glitch Explained — Wired. Detailed breakdown of the 2012 Linux hrtimer bug that took down Reddit and Qantas.
• How and why the leap second affected Cloudflare DNS — Cloudflare’s excellent, candid post-mortem of their 2017 New Year’s RRDNS outage.

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