Building a BaZi Calculator Exposed How Deeply Flawed Birth-Time Data Can Be

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A developer building a BaZi Chinese astrology calculator discovered that standard birth-certificate times are unreliable inputs because the tradition relies on solar time, not clock time. Three compounding factors drive the error: historical daylight saving rules, longitude offset from a zone's standard meridian, and the astronomical equation of time, which together can shift a birth time by over an hour. China's forgotten DST experiment from 1986 to 1991 is a key example, where a Shanghai birth in July 1988 was recorded an hour ahead of standard time with no modern lookup catching it. The developer found that Node.js's built-in Intl API, backed by the full IANA timezone database, can recover historical UTC offsets without bundling any extra timezone data. The post includes working TypeScript implementations and warns of subtle runtime bugs around midnight handling and performance costs of repeated DateTimeFormat instantiation.

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ProgrammingDEV Community ·

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