testland/dst-transition-reference
Pure-reference catalog of Daylight Saving Time (DST) transition patterns and their canonical bug classes. Covers the spring-forward (skipped hour: 02:00 → 03:00 local) and fall-back (repeated hour: 02:00 → 01:00 local) transitions, the historical irregularity of DST (different jurisdictions, transitions on different dates, some regions abolish DST or never adopted it), the IANA timezone database (tz / Olson DB) as the canonical source, and the testable behaviors DST creates (duplicate / missing local timestamps, cron jobs that fire 0 or 2 times, billing periods that miss / double-count, recurring meetings on transition days). Per-jurisdiction DST-rule tables, refreshable per-region test-data fixtures, and the leap-second reference (23:59:60 insertion, time_t stalls, leap-smear vs step, monotonic-clock fixes) live in references/. Use when designing or auditing time-handling code or test cases, or when auditing leap-second assumptions.
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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.