The Year 2038 bug blog

"Leap smear", Google Public NTP

From https://developers.google.com/time/smear

Since 2008, instead of applying leap seconds to our servers using clock steps, we have "smeared" the extra second across the hours before and after each leap. The leap smear applies to all Google services, including all our APIs.

A software library for managing this is "unsmear" which converts between smeared time and unsmeared time.

https://github.com/google/unsmear

This C++ library converts precisely between timestamps in the timescale of UTC with smeared leap seconds and the unsmeared TAI and GPS timescales.

This allows smeared time to be stored and distributed exclusively, then converted to and from other timescales in applications where the smear's 11.6 ppm frequency change is consequential. No parallel time distribution systems are required, nor is any mechanism necessary for a system to record what timescale its system clock is using.

The narrative notes that this works for both positive and negative leap seconds, and avoids steps or clock discontinuities.

Melting ice solves leap-second problem — for now. Nature 628, 273-274 (2024)

From Nature, 27 March 2024 https://www.nature.com/articles/d41586-024-00850-x https://doi.org/10.1038/d41586-024-00850-x

Humans’ effect on the polar ice sheets is slowing Earth’s rotation, posing challenges for its alignment with the official time standard. Two researchers discuss the science behind the slowdown and the impact it has on timekeeping.

The leap-second problem in question is the "negative leap second", which hasn't happened yet and is thus untested.

UTC is currently computed using data from about 450 atomic clocks, which are maintained in more than 80 institutions around the world. It is disseminated in real time by these time laboratories, by means such as radio or telephone signals, the Internet or optical fibre protocols, and also through GNSS signals. Since 1972, irregularities in Earth’s movement have called for 27 leap seconds to be added — at irregular intervals and with a maximum of only 6 months’ notice each time. The irony is that metrologists now face the challenge of removing a leap second from UTC for the first time, because Earth’s rotation is gradually getting faster than the time standard set by atomic clocks.

Why is the earth's rotational speed changing? Core-mantle coupling is likely.

The effects of tidal dissipation and shape adjustments have not changed appreciably since the advent of modern atomic timekeeping, but the impact of core–mantle coupling on Earth’s rotation varies on multiple timescales as a result of the fluid nature of the outer core. And herein lies the probable cause of timekeeping’s most recent dilemma: leap seconds have been required with much lower frequency since 2000 than in the previous 30 years, which indicates that Earth’s rotation rate is accelerating. Given the stability of tidal dissipation and shape-adjustment effects over this period, the main culprit must be core–mantle coupling.

The authors are Patrizia Tavella from BIPM https://www.bipm.org/en/ and Jerry Mitrovica from Harvard https://mitrovica.eps.harvard.edu .