How we calculate BaZi
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This page explains exactly how our BaZi calculator turns a birth time into four pillars and a reading, so you can check any result against it.
1. True solar time for all four pillars
China uses one clock time (UTC+8, set at 120°E) for the whole country, but the sun does not. Before anything else we convert your clock time to true solar time at your birthplace: first remove daylight saving time if it was in force, then correct for longitude (4 minutes per degree), then add the equation of time (the sun runs up to about 16 minutes fast or slow over the year).
All four pillars are cast from this true solar time, not only the hour pillar. Near midnight or near a solar-term boundary, the day or even the month pillar can change too.
How much it matters
We cast 2,000 random birth times (1950–2009) for each city below, once with true solar time and once with plain clock time, and counted how often the hour pillar came out different:
| Birthplace | Longitude | Hour pillar differs |
|---|---|---|
| Shanghai | 121.5°E | 6.4% |
| Beijing | 116.4°E | 10.2% |
| Harbin | 126.6°E | 23.5% |
| Chengdu | 104.1°E | 45.9% |
| Urumqi | 87.6°E | 99.5% |
The further west the birthplace, the bigger the gap. In far-western China, almost every chart cast from clock time has the wrong hour pillar.
2. Year, month, day and hour boundaries
Year changes at the exact moment of Lichun (around 4 February), not at Lunar New Year. Month changes at each of the twelve Jie solar terms, timed to the minute from an astronomical ephemeris. Day changes at true solar midnight; 23:00–24:00 is the late Zi hour and still belongs to the same day. Hour follows the twelve two-hour branches.
3. How we check the pillars
We cross-checked 5,000 random charts (born 1940–2009 at five longitudes across China) against an independent open-source calendar library given the same true solar time. All four pillars matched in 5,000 of 5,000 charts. Fixed test charts run automatically before every release.
4. How we judge strength and favourable elements
Our rule is support-or-restrain first, seasonal balance second. We first weigh how strong the Day Master is: the season it was born in, its roots in the branches, and how many stems help or drain it. A weak Day Master is supported; a strong one is restrained. That decides the useful element.
Only when the chart is clearly too cold or too hot do we bring in seasonal balance as a supplement. Seasonal balance can add a favourable element, but it never pushes an element into the unfavourable list on its own. Every conclusion on your result page shows the reasons behind it.
5. What we don't do
We don't predict illness, death, lawsuits or exact money amounts, and we don't sell remedies. Classical sayings that are harsh or absolute are rewritten in calmer terms.
FAQ
Do I need to convert to true solar time myself?
No. Enter the clock time on your birth certificate and pick the birthplace; we do the conversion, including daylight saving time in the years it applied.
I was born at 23:30. Which day is my day pillar?
We count 23:00–24:00 as the late Zi hour of the same day, so the day pillar does not change until true solar midnight. The hour pillar is already Zi.
Why does my chart differ from another site?
Usually one of three reasons: the other site uses clock time instead of true solar time, it changes the day at 23:00, or it changes the year on Lunar New Year instead of at Lichun. Check the true solar time shown on your result first.
What if I don't know my birth hour?
Leave the time empty: the year, month and day pillars and most of the structure still work. Readings that depend on the hour pillar are marked as uncertain.