Recurrence Expander

https://s1.oddments.ai/v1/expand

Turn RFC 5545 RRULEs and cron expressions into exact UTC instants. IANA timezones and daylight saving are handled correctly. The two hard cases are flagged by name. A spring-forward time that never happens is flagged nonexistent. A fall-back time that happens twice is flagged ambiguous. Results are deterministic and reproducible. Every response reports the tzdata and library versions it used. Every response also carries an Ed25519-signed reproducibility receipt. The receipt binds the input hash, the output hash and the engine versions. WHAT YOU ARE NOT BUYING: secrecy. You can expand the rule yourself and check every instant we return. The fee buys current timezone data, the two daylight-saving edge cases handled rather than assumed, and a reproducible record of which versions produced the answer. WHAT THIS DOES NOT DO, stated up front. (1) It consults no calendar, holiday feed or exception list. It expands exactly the rule you send, so a cancellation you did not encode is invisible to it. (2) Timezone answers are only as current as the tzdata version reported in every response. A zone whose rules change after that release is computed on the old rules. (3) cron is the 5-field POSIX form only, as published in the request schema. No seconds field and no vendor extensions. (4) The signature proves origin and integrity, not correctness. (5) Rules are capped at 25 per call and 500 occurrences per rule. A request over either is refused with a 400 invalid_rule before any expansion happens. Free preview endpoint at /v1/preview.