Why the place of birth and true solar time matter

The traditional calendar counts the hours of the day from the Sun, not from a clock: the double hour of 子 (the Rat hour, 23:00–01:00) is centred on local midnight, 午 (the Horse hour, 11:00–13:00) on local noon. A clock shows zone time, which is the same across a whole time zone. Between the two there is a difference that can reach more than an hour, and the hour pillar of a chart depends on it.

Two corrections

The BaziArt engine converts the civil time of birth into true solar time in two steps, by default for the day and hour pillars.

Longitude. A time zone is centred on a meridian; every degree east or west of it moves local solar time by four minutes. Madrid lies at -3.7° longitude, while its zone (UTC+1 in winter) is centred on 15° east: local solar time there runs 74.8 minutes behind the zone's clock.

The equation of time. The Earth's orbit is not circular and its axis is tilted, so the Sun runs ahead of or behind the mean clock by up to about sixteen minutes over the year. The engine uses the formula from Meeus, “Astronomical Algorithms”, chapter 28. On 21 March 1990 the Sun is a further 7.3 minutes behind the mean clock.

The same chart, two births

The example throughout these materials is a woman born on 21 March 1990 at 10:30 in Madrid; the chart was calculated by the BaziArt engine with true solar time. Nothing about this chart is special; it was chosen because it illustrates several of the rules described here.

Born at 10:30 by the clock, the person's true solar time is 09:07: about 82 minutes earlier (times are shown to the minute). Both fall in the same double hour, so the hour pillar is 辛巳 either way.

Shift the birth to 11:20 by the clock and the difference shows: true solar time is 09:57, still inside the previous double hour. With the correction the hour pillar is 辛巳; without it the engine would have written 壬午, the next pair in the cycle.

That is the whole reason the calculator asks for a place: without it, the chart of a person born at 11:20 in Madrid would carry the wrong hour pillar, and with it a possibly different strength verdict and a different luck-pillar start.

What the engine does with the place

• The time zone must be an IANA name (Europe/Madrid, America/Bogota). An unknown name is an error, not a silent fallback to UTC.
• Historical clock changes come from the time-zone database for the date of birth, including summer time where it existed.
• The place is used for the longitude only; the city name is not stored by the calculator on this site.
• The correction can be switched off in the app's calculation settings, and the choice is saved with the chart.

When the hour is near a boundary

Double hours change at odd hours of true solar time (01:00, 03:00, and so on). If the corrected time falls within a few minutes of a boundary, the app shows a notice with both candidate pillars instead of silently picking one, and the rectification tools let the hour be checked against known events. The calculation method page lists the day-boundary rule (23:00 by default) and the alternative.

Try it with your own data in the calculator: the result shows the civil time and the true solar time side by side.