UTC will become continuous in 2027, ending leap seconds
The technical actions needed to ensure the continuity of UTC
The 28th CGPM has decided that continuous UTC will take effect on 20 May 2027, with the maximum difference |UT1-UTC| allowed to reach 3600 seconds. This eliminates leap seconds, which pose high risks to critical infrastructure, especially a negative leap second with a 30% probability by 2035. The BIPM will collaborate with ITU, IAU, IUGG, IERS, and others to inform users and monitor UT1-UTC.
The additional risk of a delay in updating UTC could cause some user organizations to change their reference time scales to, for example, a GNSS time instead of UTC, seriously compromising the concept of international standardization in metrology.
- BoppreH
Context: BIPM is responsible for the UTC time standard, and in this document they decide to stop adding (or removing) leap seconds, starting on 20th of May 2027 and lasting for the foreseeable future (until one leap hour is needed, but even then it's unclear if they'll do something). Together with the decision to not add a leap second in December, the 27 leap seconds that have been added so far should be the only ones, ever.
The page lists several good reasons for this, including:
- It's a common source of bugs.
- Solar time is not that important in a world of coarse timezones.
- IoT and anything that cannot receive time updates have trouble with leap seconds.
- Negative leap seconds have never occurred before, but might occur in the future, and almost no software has tested this case.
I'd also add:
- Unix timestamps are actually defined as "seconds since epoch minus however many leap seconds have happened". It's a hack to keep code that assumes "86400 seconds in a day" from breaking. It means that during a leap second the Unix timestamp goes back and repeats a previous value, making them ambiguous and non-monotonic.
- Leap second unpredictability can make it impossible to calculate how many seconds are between two UTC dates or Unix timestamps, for example the countdown to the year 2038 bug.
- The proper ISO8601 formatting of leap seconds ("23:59:60") is widely unsupported, breaking parsers, along with the assumption of how many seconds are in a minute, hour, or day.
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EDIT […]
- amelius
AI has become real, and we're cancelling the leap seconds?
This is unfair.
If AI wants to be a coder then it has to do leap seconds. In fact, let's give it leap minutes and leap hours too.
- frangonf
Goodbye sundial welcome cesium clock. Astrology app devs: remember to carry the drift in Temporal for the Ascendant, specially around 2267 you might get your love and money life predictions messed up.
- neutrinobro
Yay! No more leap seconds to manage! This is going to make my life much easier.
Edit...scratch that, now I have to keep track of UT1-UTC as a separately continuously varying parameter provided by IERS instead of tzdata
- hnuser123456
A practical solution. As long as we can keep 12:00 near high noon and 00:00 near "high midnight(?)", that should be good enough.