A BaZi chart is only as accurate as its boundaries. EightLetters computes every boundary from second-precision astronomical solar term data rather than lookup tables or the lunar calendar, and applies a true solar time correction when a birth city is provided.
The year pillar changes at the solar term Start of Spring (立春), typically around February 4th — not at Chinese New Year, which is a lunar calendar event that can fall before or after it. Someone born between Chinese New Year and Start of Spring is still assigned the previous year's stem-branch pair. This single distinction is the most common source of incorrect charts online.
The month pillar is bounded by the 24 solar terms (specifically the 12 "jie" terms, not the "qi" terms), not by the lunar calendar month. We compute solar term timing to the second using the sxtwl astronomical calendar library rather than a static lookup table, so the exact hour and minute a month boundary falls on is calculated for the actual year in question, not approximated.
The day pillar follows the traditional sexagenary (60-cycle) count, with the day changing at 23:00 — not midnight. A birth time of 23:15 is assigned the next day's day pillar, even though the hour branch is still Zi (子).
The hour pillar's stem is derived from the day pillar's stem using the classical Five Rats method (五鼠遁) — a fixed lookup table mapping day stem to the stem of the Zi hour, from which the rest of the day's twelve hour stems follow in sequence.
Clock time is civil time, standardized to a time zone's meridian — it isn't the same as true solar time, which BaZi is traditionally based on. When you provide a birth city, we look up its coordinates, resolve its IANA time zone (DST-aware), and apply a correction of (longitude − time zone's standard meridian × 15) × 4 minutes. A birth city near the eastern or western edge of a wide time zone can see a correction of 30–60 minutes or more — enough to shift the hour pillar.
Your first luck pillar's starting age is derived from the number of days between your birth and the nearest solar term, converted to age at roughly 3 days per year. Whether that count runs forward to the next solar term or backward to the previous one depends on your gender and the yin/yang polarity of your birth year's heavenly stem — the same rule that determines whether your luck pillars advance or reverse through the 60-term cycle from your month pillar.
See these rules applied to your own birth data.
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