rebar-rrule

LLMs guess recurrence math from memory and get it wrong in predictable ways:

• DST transitions. "Daily at 9am New York" is NOT "every 24 hours" — the UTC offset shifts from -05:00 to -04:00 at the spring-forward. These tools keep the wall clock fixed and move the offset, like a calendar does.
• BYDAY arithmetic — "the second Tuesday of each month" or "every other Friday from Jan 2" across month boundaries is exactly the off-by-one territory models hallucinate in.
• It crashes — or worse, hangs — on bad input. The underlying library throws on malformed rules with internals-leaking messages, silently mis-evaluates others (INTERVAL=0 yields nothing, COUNT=-2 acts unbounded, BYMONTHDAY=99 matches nothing), and infinite-loops on INTERVAL=abc. Agent-assembled rule strings are exactly this kind of input. Every tool here validates first and returns an actionable error instead of a 500 — and refuses pathological scans instead of stalling.
• Unbounded rules. FREQ=DAILY recurs forever. Expansion is always capped (default 100, max 1000 occurrences) with an explicit truncated flag, so a runaway rule can't blow up a context window.