Should a BaZi chart use clock time or true solar time?
Meridian uses true solar time, because the hour branches were defined by where the sun actually is rather than by a time zone drawn up in the last century and a half. Not every school agrees, and in parts of Asia the two differ by more than an hour, which is enough to move the hour pillar.
Why the clock is not the sun
Time zones are administrative. They apply one offset across a wide band of longitude so that everyone in a country keeps the same clock. Your local noon, the moment the sun is highest where you stand, drifts from clock noon by four minutes for every degree you sit away from the zone meridian.
China runs a single zone across roughly sixty degrees of longitude. Spain keeps central European time while sitting west of Greenwich. The gap between clock and sun is not a rounding error.
The size of it
| Born 15 June 1990, 07:30 clock, UTC+8 | Singapore | Hong Kong |
|---|---|---|
| Longitude | 103.82°E | 114.17°E |
| True solar time | 06:25 | 07:06 |
| Hour pillar | 辛卯 | 壬辰 |
Singapore's clock sits about 65 minutes ahead of its sun, an unusually large gap created when it moved onto its neighbour's zone in 1982. Anyone born there whose clock time lands near the edge of a two hour block has an hour pillar that depends entirely on whether the correction was applied.
The second correction
Longitude is the large part. The equation of time is the small one. The earth's orbit is an ellipse and its axis is tilted, so solar days are not all the same length. Across a year the sun runs up to about sixteen minutes ahead of or behind the clock, even at your zone meridian.
It rarely decides a pillar on its own. It decides plenty of them in combination with longitude.
Both corrections, every time, from your birth place rather than your time zone. The chart shows the clock time you gave and the solar time it was built on, so nothing is happening behind your back. If your hour pillar sits near a boundary, Meridian will tell you, because a fragile pillar is something you should know about before you build an interpretation on it.
Schools differ on this
Correcting to the sun is the majority position in classical practice and it is what Meridian does. It is not unanimous. Some modern practitioners work from civil clock time on the grounds that it is what the birth record states and what the person has lived by, and some apply longitude but not the equation of time.
A chart that does not say which it used cannot be compared with one that does. That is the part worth insisting on, whichever convention you end up preferring.
Who should care most
- Anyone born in a country running one time zone across a wide span, such as China, India or Spain.
- Anyone born in Singapore or Malaysia, where the standard offset moved.
- Anyone whose clock time falls within about twenty minutes of an odd hour, since that is where the branches change.
Sources
Related
- Why do two BaZi calculators give you different charts?
- Born overseas: which time does your chart use?
- Does daylight saving change your hour pillar?
See your chart in clock time and true solar time
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.