The Hard Part of a Global Birth-Chart Calculator Was Time

A birth-chart form looks simple: ask for a date, time, and place, then calculate. The interface may be simple. The input is not. 1992-11-01 01:30 does not identify one universal instant. It is a wall-clock reading that only becomes meaningful after you resolve the place, the historical time-zone rule, and any daylight-saving transition. If the time is unknown, inventing a convenient default can create chart features that were never supported by the user’s data. I ran into these problems while building AstroZen , a Next.js application that calculates a BaZi Four Pillars chart and a Western natal chart before generating an optional interpretation. The most important architectural decision was this: Calculation is an evidence pipeline. Interpretation is a separate layer. This post explains the calculation pipeline, the failure modes I had to remove, and why “unknown” must remain unknown. 1. A city name is not a coordinate Early prototypes often use a short city list or a default coordinate. That works for a layout demo, but it is not acceptable once location affects the result. Names are ambiguous: Paris can mean France or Texas. Springfield needs a state or region. Country abbreviations come in several forms. A valid city must resolve to both coordinates and a time-zone identifier. AstroZen sends the submitted city to Open-Meteo’s geocoding service, then scores the returned candidates against the requested country and optional region hint. It keeps the following structured result: type ResolvedPlace = { name : string ; region : string | null ; country : string ; countryCode : string ; latitude : number ; longitude : number ; timeZone : string ; // IANA, for example "Europe/Madrid" }; Candidate selection considers exact city-name matches, country matches, an optional region hint, and population as a small tie-breaker. Population never replaces the country check. The more important rule is what happens when resolution fails: if ( ! selected || ! countryMatches ( selecte