Why do two BaZi calculators give you different charts?
Almost always one of four things. How the calculator handles true solar time, where it puts the day boundary, whether it corrects for daylight saving, and what it does when the birth time is unknown. Each is a choice rather than a bug. Two correct calculators can disagree.
A worked example
Same date. Same clock time. Same time zone. Only the city changes.
| Born 15 June 1990, 07:30, UTC+8 | Singapore (103.82°E) | Hong Kong (114.17°E) |
|---|---|---|
| True solar time | 06:25 | 07:06 |
| Day pillar | 辛亥 | 辛亥 |
| Hour pillar | 辛卯 | 壬辰 |
The hour pillar moves because the two cities sit at different longitudes inside one shared time zone. Singapore's clock runs about 65 minutes ahead of its own sun, so 07:30 on the clock is 06:25 by the sun, which falls in the 卯 hour. Hong Kong's 07:30 is 07:06 solar, which falls in 辰.
A quarter of the chart is different and neither calculator is wrong. One applied the longitude correction and the other did not.
Meridian corrects to true solar time by default, from your birth longitude. You can switch the correction off and watch which pillars move, which is usually the fastest way to understand why a chart you were given elsewhere looks different. Every chart records the conventions that produced it, so you can always compare like with like.
The other three, briefly
The day boundary. The 子 hour runs 23:00 to 01:00 and crosses midnight. One school turns the day at 23:00, another at midnight. A birth in that window can get a different day pillar, which changes the day master.
Daylight saving. Hong Kong used summer time on and off until 1979. An hour of uncorrected clock error moves the hour pillar by a full branch.
Unknown birth time. Some calculators quietly default to noon and hand you a complete chart. The hour pillar in it is a guess that looks exactly like a fact.
Four questions for any calculator
- Does it use true solar time, and does it ask for a birth place rather than only a time zone?
- Where does it put the day boundary?
- Does it apply historical daylight saving for that year and country?
- What does it do when the birth time is unknown?
A tool that cannot answer these is not necessarily wrong. It is just not telling you what it did.
Sources
- Hong Kong Observatory, local mean solar time and the equation of time
- Hong Kong Observatory, summer time in Hong Kong 1941 to 1979
Related
- Should a BaZi chart use clock time or true solar time?
- Born overseas: which time does your chart use?
- Does daylight saving change your hour pillar?
Cast your chart and see both conventions side by side
Worked example computed by Meridian's chart engines on 1 August 2026. The engines are deterministic, so the same input returns the same chart.
This page explains calculation conventions. It does not interpret a chart and is not advice.