Your birth time is lying to you: a time-zone rabbit hole in a Chinese astrology calculator

A Chinese astrology calculator for BaZi, or Four Pillars birth charts, reveals a time-zone rabbit hole. Birth chart calculations rely on solar time, which differs from clock time. Three factors contribute to this discrepancy: daylight saving time, longitude, and the equation of time. For instance, a July birth in Vancouver requires a roughly 78-minute correction.

To calculate the offset from UTC for a specific time zone, the code utilizes the Intl.DateTimeFormat API, which ships the full IANA tzdb, including historical oddities. By formatting a UTC instant in any zone and parsing the formatted parts, the offset can be obtained without bundling tz data.

A subtler issue arises when determining the standard offset due to Daylight Saving Time (DST), which lacks a dedicated API in JavaScript. The code employs a heuristic by sampling the zone's offset on specific dates (January 1, July 1, and the birth instant) and selecting the minimum value as the standard offset.

One edge case is the date line, where the naive calculation using longitude × 4 minutes from Greenwich results in a full-day error in the local mean solar time. To address this, the code normalizes the longitude to within ±180° of the zone's standard meridian.

Another edge case is Morocco's negative DST during Ramadan, which the heuristic addresses by taking the minimum offset among sample dates. The three components—the date, time, and DST—can be displayed in an addition table, revealing the complex relationship between time zones and BaZi chart calculations.