testland/dst-transition-reference
Use when designing or auditing time-handling code or its tests around daylight saving time: skipped and repeated hours, jurisdiction differences, the IANA tz database, cron jobs that fire zero or two times, billing and meetings on transition days, plus leap seconds and clock smearing.
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smear-strategies-and-history.mdreferences/
Leap-smear strategies and the historical leap-second record
Deep reference for leap-seconds.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.