Bazi Explained
What is True Solar Time?
True Solar Time (sometimes called Local Apparent Time) is the time the sun actually sees at a given location — the moment the sun crosses the meridian overhead is solar noon, and other times are measured from that. Clock time is a convenience built on top of time zones; solar time is the real thing the sun cares about. For systems like Bazi, which were developed centuries before time zones existed, solar time is the right reference.
Why clock time differs from solar time
Two effects shift the gap between clock time and solar time. The first is the longitude offset: time zones are blocks of 15°, but the sun moves continuously. A city near the eastern edge of its time zone sees solar noon earlier than the clock reads; a city near the western edge sees it later. The second is the Equation of Time, a correction for the fact that the Earth's orbit is elliptical and its axis tilted — the sun doesn't move at a perfectly even pace across the sky through the year. The combined adjustment can be tens of minutes.
A concrete example
Take a baby born in San Francisco at 11:00 PM clock time on November 1st. San Francisco sits at about 122.4° W, while Pacific Standard Time's central meridian is 120° W, so the longitude offset alone subtracts about 10 minutes. Add an Equation of Time correction of roughly +16 minutes for early November, and the True Solar Time becomes around 11:06 PM. That's on the boundary between the Hài (亥) Hour and the Zǐ (子) Hour — the moment crosses into the next day's Day Pillar in Bazi. Without the correction, the entire chart shifts by one day.
Why this matters for the Hour Pillar
The Bazi Hour Pillar uses twelve two-hour windows, each named after an Earthly Branch (Zǐ, Chǒu, Yín, Mǎo, and so on). A birth recorded near the boundary of one of those windows is the most likely to shift when corrected to True Solar Time. The Hour Pillar in turn determines the Hour Stem, which affects Day Master strength, the Ten Gods analysis, and the entire daily reading. A 20-minute correction can change the whole interpretation.
How True Solar Time is calculated
The standard recipe: convert the recorded time back to standard local time (undo daylight saving), subtract or add the longitude offset to the time zone's central meridian (4 minutes per degree), and apply the Equation of Time for the date. Modern astronomy libraries compute the Equation of Time from the date directly. Most casual Bazi apps skip all of this and use the user's clock time as-is, which produces charts that drift from the classical method.
Frequently asked questions
How much can True Solar Time differ from clock time?
Commonly 0 to 30 minutes, occasionally more. The exact difference depends on how far the birth location sits from the time zone's central meridian (longitude offset) and the time of year (Equation of Time). In some places the gap can briefly exceed 45 minutes when both effects line up.
Why does longitude matter?
Time zones step in 15° increments, but the sun moves continuously. A city sitting at the western edge of a time zone sees solar noon roughly 30 minutes after the clock noon; a city at the eastern edge sees it about 30 minutes before. Bazi predates time zones — it cares about the sun, not the clock.
What is the Equation of Time?
The Earth's orbit is elliptical and its axis is tilted, so the sun's apparent speed across the sky varies through the year. Solar noon can drift up to roughly 16 minutes earlier or later than clock noon depending on the date. A sundial in February runs about 14 minutes slow; in November it runs about 16 minutes fast.
Why does this matter for Bazi?
Bazi Hour Pillars are two-hour windows aligned to the sun, not the clock. A birth time near the boundary of an hour can shift to the next Earthly Branch when corrected for True Solar Time, which changes the Hour Pillar character and therefore the reading.
Does this matter for Western astrology too?
Less than for Bazi, because Western charts use degree positions of planets that change slowly, but it still affects the Ascendant and house cusps, which are time-sensitive. Most serious Western astrologers also correct to True Local Time.
What if I don't know my exact birth time?
Use the best estimate you have. If only the date is known, most Bazi calculators default the Hour Pillar to noon — Year, Month, and Day pillars stay accurate, but the Hour Pillar interpretation should be treated as provisional.
Does daylight saving need to be undone?
Yes. The first step of any True Solar Time calculation is to convert the recorded clock time back to standard local time, then apply the longitude offset and Equation of Time. A serious Bazi calculator handles this automatically.
A free Bazi calculator that applies longitude and Equation of Time corrections is available on this site.
Related reading: What is your Day Master? · The Five Elements explained · What is Bazi?