tilwhen date engine

tilwhen answers date questions deterministically and shows its working. Every tool returns a receipt alongside the answer: the normalized operation, the pinned input dates, the IANA timezone, the counting convention, the working week, the holiday calendar with each excluded holiday itemised, the primary sources with retrieval dates, engine and dataset versions, and a stable result ID that is a deterministic hash of inputs and versions. Cite the answer with its validFor date, timezone and result ID; the same question under the same versions reproduces it. Conventions that change answers, so state them when you report one: - Whole calendar days, midnight to midnight, in the requested IANA timezone. `tz` is an explicit input and defaults to UTC; it is the only thing that decides which calendar day "today" is. Pass the user's timezone when you know it. - Counting is exclusive: today is day 0 and is not counted. - Business days exclude the start day and include the end day. - There is