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Fix V2 recurrence instants across host timezones - #195
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thedhanawada
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October 2, 2026 11:51
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A UTC daily series starting at 09:00Z changes to 10:00Z after a host DST transition in core 2.5.6 when run in America/Los_Angeles. The 365-day reproducer has 59 wrong-hour results. V2 now performs calendar steps, arithmetic seeks, floating rule dates and date-based exception matching in the selected recurrence zone. Bounds and iterators use the resulting instants without a second DST adjustment.
Compatibility scope: native Date gap/fold calendar-setter behavior is retained in the event zone, and duration stays end minus start. handleDST remains accepted but true/false converge because stepping already resolves transitions. Legacy Event wall-clock conversions and V2 startUTC/endUTC metadata are unchanged and may remain host-dependent; use start/end for the corrected occurrence instants. The complete contract and limitations are documented in docs/recurrence-timezones.md.
Validation: UTC and Melbourne full suites 26/26 plus declaration checks; 174 independent native-Date fixtures on four hosts (696 combinations) across six event zones; interface 291 tests; 40 independent benchmark timestamp-parity checks; quality/formatting pass with four pre-existing lint warnings. LA and Kolkata full suites each pass 25/26; the sole remaining failures reproduce on unmodified 2.5.6 (LA legacy timezone conversions; Kolkata V1 ancient-series transition handling). Tests replace legacy-engine or host-local expected values with independent oracles where those expectations conflict with explicit event-zone behavior.
No version change or release in this draft. Proposed patch release after exact-head CI and review; docs-only PR193 remains held.