ADR-0003: Chrono, IANA zones and validated half-open intervals
Date: 2026-09-19 Status: Accepted (Foundation implementation within specification 0.4)
Context
Instant identity and DST behavior must not rely on manually maintained timezone arithmetic.
Decision
Use chrono without clock features and chrono-tz with the bundled IANA database. Persist UTC instants plus an explicit original timezone. TimeRange has private bounds and validating serde conversion. Duration is positive integral seconds. Local resolution rejects ambiguous/nonexistent times; callers must resolve them before planning. Recurrence remains a bounded expansion trait.
Alternatives
A custom zone engine is unsafe; silently selecting either DST fold loses meaning; allowing naive datetimes inside TimeRange weakens identity.
Consequences
Timezone database updates arrive via dependency updates. No hidden system clock. ExplicitDate currently constrains the local start date. Subsecond instants are representable; candidate steps/durations use integral seconds.
Migration / Compatibility
Temporal serde validation is part of CPIR 0.1. No change to the Master's half-open convention.
References
Master section 6; Testing standard.
2026-09-20 follow-up
Bounded daily/weekly recurrence is implemented by ADR-0009; plain local conversion still rejects gaps/folds.