Why true solar time matters for your BaZi chart

The hour pillar of a BaZi chart divides the day into twelve two-hour periods, 子 to 亥, and the tradition ties them to the sun: the 午 hour is centred on noon, when the sun stands highest. Your clock doesn't show that time. It shows the time of a whole zone, often more than a thousand kilometres wide and sometimes shifted by daylight saving. True solar time (真太陽時) removes those shifts and asks what the sun was actually doing where you were born.

Why the clock and the sun disagree

There are three separate reasons.

• Longitude. A time zone takes its time from one meridian: UTC+8 from 120° east, UTC+1 from 15° east. The sun crosses one degree of longitude every 4 minutes, so a birthplace east of the zone's meridian reaches noon before the clock does, and one to the west reaches it later.
• The equation of time (均時差). Even on the meridian itself, sundial time runs ahead of and behind clock time over the year, because the Earth's orbit is an ellipse and its axis is tilted. The gap reaches about 14 minutes behind in mid-February and about 16 minutes ahead in early November.
• Daylight saving. Summer time sets the clock an hour ahead of the zone's standard time, so the sun falls a further hour behind the clock.

How the calculator corrects it

Our BaZi calculator works through three steps:

• Clock time to an instant. Your birth date and time are read in your birthplace's time zone with its historical rules, so any daylight saving in force at the time is included. China, for example, kept summer time from 1986 to 1991. If a clock time never existed (the clocks jumped forward) or happened twice (they went back), the chart says how it was read.
• Longitude correction. Four minutes per degree, measured against the offset actually in force, so the daylight-saving hour comes out at this step.
• Equation of time. Added for that date from a standard astronomical formula.

The result is local apparent solar time, and the day and hour pillars are read from it. The chart lists each correction in minutes.

What the numbers look like

• Beijing (116.4° E) lies about 3.6° west of the UTC+8 meridian, so the longitude correction is about −14 minutes. With the equation of time, a clock noon becomes about 11:31 in mid-February and about 12:02 in early November.
• Ürümqi (87.6° E) keeps Beijing time, more than 32° from its meridian. The correction is over two hours, and a clock noon in February is around 09:36 by the sun.
• Madrid (3.7° W) uses UTC+1, and UTC+2 in summer. On a July day the correction comes to about two and a quarter hours, and a clock noon reads about 09:39 by the sun.

What it changes in a chart

• The hour pillar. The two-hour boundaries fall on odd hours (13:00, 15:00 and so on). A shift of fifteen minutes can cross one; a shift of two hours always does. In the Madrid example, a clock noon in the 午 hour becomes the 巳 hour.
• Sometimes the day pillar. Near midnight, the correction can move a birth into the previous or the next day, and the day pillar holds your Day Master. A birth at 00:30 summer time in Madrid is 22:09 the evening before by the sun, so the day pillar and the hour pillar both change. The other way round, a birth at 22:50 in Shanghai on 3 November reads 23:12 by the sun, already the 子 hour; under the default rule that the day changes at 23:00, it takes the next day's pillar.
• Not the year, month or luck pillars. These are set by solar terms, which are single instants worldwide. The calculator compares those instants with the moment of birth directly, so local solar time leaves them alone. See how luck pillars are calculated.

When it isn't applied

True solar time needs both a birthplace longitude and a birth time. Without a place, the chart uses clock time and says so; without a time, the hour pillar is left out. You can also switch the correction off in the advanced settings if you prefer clock time, as some practitioners do. The zodiac calculator has no need for it, since the animal turns at a single instant, and the I Ching and Plum Blossom tools read the local clock time of the moment you cast.

Arithmetic, not prose

All of these steps are calculations done in code before the chart is drawn, and the chart lists them so you can check them. The AI only writes about the finished pillars; it has no say in the time it was given.