Unknown Time Is Not Noon: Modeling Missing Temporal Data Without Inventing Facts

Missing data is not the same thing as a convenient default. That sounds obvious, yet temporal software regularly converts an empty time field into midnight, noon, the current time, or the start of a day. The interface may look complete after that conversion, but the program has silently changed an unknown fact into a known one. This matters anywhere an hour can change the result: medical timelines, transport schedules, legal deadlines, astronomical calculations, historical records, and calendrical systems. I encountered the problem while working with a BaZi calculation pipeline. A BaZi chart can use year, month, day, and hour components. If the birth time is absent, the honest result is a three-component analysis with hour-dependent conclusions withheld. Inserting noon would make the output look richer while making its provenance weaker. The useful engineering question is not “Which fallback time should we choose?” It is “How do we keep uncertainty visible through every layer of the system?” The public calculation evidence repository provides the concrete calendar-domain fixtures referenced below. The rest of this article focuses on the reusable software boundary behind them. Model knowledge, not just a string A common input model makes absence too easy to erase: const birthTime = form . time || " 12:00 " ; After this line runs, downstream code cannot tell whether noon came from the user or the fallback. Validation, analytics, caching, and the result renderer all see the same string. The information loss happens before the calculation begins. A small discriminated union keeps the two states separate: /** * @typedef {{ kind: "known", localTime: string, source: "user" }} * KnownTime * @typedef {{ kind: "unknown" }} UnknownTime * @typedef {KnownTime | UnknownTime} BirthTime */ function parseBirthTime ( value ) { const normalized = value ?. trim (); return normalized ? { kind : " known " , localTime : normalized , source : " user " } : { kind : " unknown " }; } This typ