testland/leap-second-reference
Pure-reference catalog of leap-second mechanics and the bugs they cause: the 23:59:60 UTC insertion (announced ~6 months ahead by IERS Bulletin C; 27 inserted 1972-2016; abolished by 2035 per CGPM 2022), the Google/AWS leap-smear alternative, and the four bug classes a real insertion exposes - time_t non-monotonicity, negative durations, NTP cascading, and cross-node clock skew - each with a monotonic-clock fix and a freezegun simulation. Use when auditing time-sensitive code (financial timestamping, distributed logs, NTP-driven schedulers) for second-by-second progress assumptions; for the far more common daylight-saving-time transition hazards, use dst-transition-reference instead.
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smear-strategies-and-history.mdreferences/
Leap-smear strategies and the historical leap-second record
Deep reference for leap-second-reference SKILL.md. Consult when
comparing how platforms absorb a leap second and when you need the
factual insertion history.
Leap-smear
Per Google's "Time, technology and leaping seconds": googleblog.blogspot.com/2011/09/time-technology-and-leaping-seconds.html, Google "smears" the leap second rather than stepping the clock. The published standard is a "24-hour linear smear from noon to noon UTC" (Google Public NTP: Leap Smear), adding a small fraction to each second so the total adds up to 1 second of slowdown, with no actual 23:59:60.
Leap second strategy comparison:
| Approach | What happens |
|-----------------------|------------------------------------|
| IERS spec | 23:59:60 UTC inserted (real second)|
| Linux kernel default | Real insertion; time_t stalls 1s |
| Google leap-smear | Distributed over 24h |
| AWS leap-smear | Linear over 24h |
| NTP "step" | Jump 1s; subsequent time_t differs |The smear is operationally invisible to applications; the spec exposes the discontinuity.
Historical leap seconds
Per IERS, 27 leap seconds were inserted between 1972 and 2026. Most recent: 2016-12-31 23:59:60 UTC. None have been added since: IERS Bulletin C 72 (6 July 2026) states "from 2017 January 1, 0h UTC, until further notice : UTC-TAI = -37 s" and that "NO leap second will be introduced at the end of December 2026" (datacenter.iers.org, Bulletin C). None expected before 2035 abolition.