Things to know about leap seconds and your Unix system

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Let me start with a small apology for not writing anything new in several months. It’s amazing it’s been so long since my last post. Anyway, June 30th is approaching quickly and this past week I’ve been in charge of reviewing the code in our ATC system regarding the possible operational impact that the next leap second will have for controllers. Fortunately, some changes had been introduced for the previous leap second in 2008 to fix a few problems that did appear in the 2005 one, and this means the impact should be minimal and it will still be safe to fly over the Iberian Peninsula during the leap second midnight. ;)

Seeing me work on that, a few colleagues and friends have asked me what the leap second thing is about and if there’s anything they need to know or do on their home systems to handle it properly. The short answer is "No". Most people won’t need to do a single thing but I’m sure some of them would enjoy a longer explanation, so here it goes.

What is a leap second?

For humans and regarding this topic, there are basically two ways to count time. On the one hand you have systems which are based on  the International System of Units. In these systems, a second is defined as a very specific amount of fixed time related to the number of periods of a specific radiation related to caesium 133 atoms. No more, no less. Some time scales, like International Atomic Time or GPS Time follow that definition and time, in them, advances at a fixed pace forward. On the other hand, we humans like to give our time scales an astronomical meaning in daily use. The Earth moves around the Sun and completes one lap in a specific period of time and we define the duration of one day and, for that, one second, regarding that movement. The UTC time scale, in which our time zones are based, works like that.

The problem with giving that astronomical meaning is that the time the Earth takes to go around the Sun is not constant, so the definition of one second according to those two scales doesn’t match exactly. But as human as we are, we decided to simplify things and define UTC time regarding a simpler, non-astronomical scale, like International Atomic Time. To preserve the astronomical meaning of UTC, then, once in a blue moon we are forced to have a minute with 59 or 61 seconds (until now, only the latter) just like from time to time the year needs to have 366 days. The moment these leap seconds are introduced is decided on the go by the International Earth Rotation and Reference Systems Service according to observations, but the event always takes place after or before 23:59:59 in June 30th or December 31st in a given year in the UTC time "zone".

Since they were introduced, International Atomic Time and UTC time have fallen 34 seconds apart and, after June 30th, the difference will be 35 seconds.

How is a leap second introduced in theory?

In theory it’s very simple if we represent time as text or as a structure containing different pieces. If what we’re doing is adding a second, in the night of June 30th we’d have:

June 30th, 2012 23:59:57
June 30th, 2012 23:59:58
June 30th, 2012 23:59:59
June 30th, 2012 23:59:60 -- leap second here
July 1st,  2012 00:00:00
July 1st,  2012 00:00:01
July 1st,  2012 00:00:02

How is a leap second introduced in practice, in a computer?

In Unix systems (and other types of systems as well) there are a lot of system calls and different APIs to handle time in different and flexible ways. Unfortunately, most of them were not designed with leap seconds in mind, specially the most interesting ones from a computational point of view because they are mainly based on Unix time. As we cannot easily represent going from 23:59:59 to 23:59:60 and then to 00:00:00, in most systems the leap second is introduced by advancing or putting back the clock one second at 23:59:59 like you’d do on a digital watch. So you’d be at the end of 23:59:59 and then you jump back to 23:59:59 again. Some other systems prefer to "freeze" the clock for one second so an application requesting the system time would not see a jump back in time between to consecutive system calls.

Anyway, it’s a bit easier to make leap-second aware those applications that need it than to introduce a new standard API that would take them into account and be used by everyone. Because, honestly, most applications don’t care about leap seconds and, for those that do, it’s not that hard to detect a leap second happened and handle the special case properly with a few lines of code.

Enter NTP

NTP or Network Time Protocol is a tool that can be used by computers to stay in sync and properly on time. Specifically, it allows to computers to talk and exchange information about what time it is allowing one of them, the client, to closely follow the time of the other one, the server, which is considered a trustworthy reference. This server in turn may be in sync with another more trustworthy time source, and so on. Obviously, this cannot follow indefinitely and at one point in time someone will have to trust a time source as being infallible.

NTP is needed because, even if computers have a precise internal hardware oscillator and clock, just like a normal watch it will go too fast or too slow. The NTP daemons running in the computer are able to measure this drift and inform the operating system kernel about it, so that system time advances at the proper pace and your clock stays in time.

The thing about NTP and leap seconds is that the protocol is leap-second aware. If a time source knows a leap second is about to take place, it can inform its clients about the upcoming leap second. These clients may in turn be servers to other clients, and they would propagate the leap second warning to them. The NTP daemon may inform the kernel about the upcoming leap second so the kernel applies the leap second jump, or may apply the jump itself if its configuration allows it to behave that way and it’s being run with enough privileges. Anyway, you probably won’t decide that as the operating system vendor will probably ship you the NTP daemon with a working configuration in that regard, so you only have to customize the reference server(s) if any or some security related parameters about who can query or modify the daemon state.

So, hopefully, if you have a Unix system synced with NTP, your computer will most likely receive a leap second warning before it happens and will make the clock jump for you without doing anything in the proper moment at midnight.

A few remaining questions

Who provides the original leap second warning? Many infallible time sources in NTP are connected to a GPS clock. This clock receives the current time from the satellites and the known offset between GPS time and UTC time. This difference will change after a leap second happens. They will also receive a leap second warning some time before it happens. Moreover, a properly configured NTP server (either for public use or for internal corporate or home use) should have its software or firmware updated so it knows in advance the leap second is going to take place, without having to wait for the upcoming modified UTF offset message from the satellites or the leap second warning. For example, if you use Linux and the NTP server from, you can download a file from the NIST FTP server with an up-to-date list of leap seconds, save it to your hard drive and configure the NTP daemon with a "leapfile" directive in the configuration file to point the daemon to the list of known leap seconds. You can even do this if you don’t trust your upstream servers are going to properly propagate the leap second warning. More information can be found in the[ wiki].

What if my computer is turned off before the leap second warning and turned on after it has happened? Nothing to worry about. At any moment the NTP daemon detects you’re off by about one second, it will step the clock to the proper time without affecting the drift calculations. For example, the manpage for ntpd in my system states the step will happen if it detects you’re off by more than 128 milliseconds.

Experiment for the future: be playing a video or a song in your computer in the moment the leap second happens and see if it stalls for one second or some other weird thing happens, to test your movie or audio player is able to detect the leap second. File a bug if it’s not. :)

As you can see, keeping your computers in sync is really easy even in the event of a leap second. It becomes a bit trickier when the system has several different devices of different brands and ages and each one of them takes the time reference from a different source, but most home computer owners don’t need to do anything special, and most people simply won’t care about the leap second anyway.

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