Should you use true solar time?

The birth time you enter is usually Beijing time from the clock. Beijing time uses longitude 120° east (near Hangzhou and Shanghai) as its standard meridian, and the whole country shares one set of clock times. But the sun runs on the local meridian: every degree of longitude is four minutes of local solar time.

Typical differences: Ürümqi (about 87°36′ east) runs roughly 130 minutes — two hours and ten minutes — behind Beijing time; Chengdu (about 104°04′ east) about 64 minutes behind; Xi'an (about 108°56′ east) about 44 minutes behind; while Shanghai (about 121°28′ east) is only about 6 minutes ahead.

The hour pillar in traditional stem-and-branch charting follows the natural hours of the sun's direct overhead position and its rising and setting, not the modern administrative time zone. True solar time correction restores the clock time to the local sundial reading, using the longitude of the birthplace and the equation of time, and then decides which hour branch it falls in.

Stem-and-branch hours switch every two hours. True solar time only actually changes your BaZi chart when the converted moment crosses a branch boundary.

A case where the hour pillar clearly changes: born in Chengdu, Sichuan, with a clock time of 09:15 (in the dragon hour, 09:00–11:00). Switch on true solar time and the 64 minutes come off, putting the real solar time at 08:11 — inside the rabbit hour of 07:00–09:00 — so the hour pillar's stem and branch change from dragon to rabbit.

A case where it makes no difference at all: born in Hangzhou or Shanghai, Zhejiang, at a clock time of 10:00. After the correction only a few minutes are involved, so the moment still falls inside the dragon hour and all four pillars are unchanged.

The case to watch most closely is the day-boundary link: born in Xi'an, Shaanxi, at a clock time of 23:20 (which by the clock is the rat hour of the next day). Subtracting 44 minutes gives 22:36, which falls back into the pig hour of the same day. In that case not only does the hour pillar change — the day pillar belongs to the previous day, and the Day Master and the Ten Gods switch with it.

True solar time depends strictly on the longitude of a particular city. If the birth city is uncertain and you pick one at random, the correction is likely to go in the wrong direction and spoil an hour pillar that was right.

If you know only the province and not the prefecture-level city, chart once with the clock time, then compare against the provincial capital's longitude to see whether the hour sits on a pillar boundary. If the hour pillar changes, mark it as “hour pillar to be verified” next to the conclusion.

If you have no idea at all about the province or city, leave true solar time off and use the clock time. The hour already carries a range of ambiguity; piling on a wrong longitude only moves the chart further from reality.

If the branch of the hour pillar has not changed after the correction, the strength of the Day Master, the generation and overcoming of the elements, the configuration of the Ten Gods and the course of the luck cycles all stay exactly as they were — just read the chart as it is.

If the hour pillar has changed, focus first on the Ten God and palace that the hour pillar's stem and branch stand for. The hour pillar mainly concerns the later years, the bond with children, your deepest personal aims and your unconscious expression, while the year and month pillars — the early foundation and the parents' career — are not affected by the hour.

For charts of births close to 23:00, where the rat and pig hours meet, always check at the same time whether the day pillar rolls over when the hour pillar changes.