True solar time: what it is and why BaZi charting cares
True solar time corrects clock time for the longitude of the birthplace and the equation of time. It can move a birth into another hour branch, and near midnight it can even touch the day pillar. Whether to use it is a stated convention, not a hidden default.
Direct answer
True solar time corrects clock time using the longitude of the birthplace and the equation of time. It can move a birth into a different hour branch. Whether a tool applies it is a convention that should be shown openly, not switched silently.
Clock time is not solar time
'True solar time' sounds like an occult term, but it is first an astronomical question and only then a charting convention. Clock time is set for administrative convenience. The position of the sun in the sky depends on the longitude of the birthplace. The two are not the same thing.
In China, for example, the whole country uses one time zone, while the land spans a wide range from east to west. Sunrise, noon and sunset do not occur at the same moment on the coast and in the interior. A child born in the west when the clock says noon may be born before the sun has reached its highest point. If the unified clock time is taken directly as local solar time, the hour pillar can land in the wrong branch.
The correction has two parts. The first is the longitude difference: the farther the birthplace is from the standard meridian, the larger the gap between local time and clock time. The second is the equation of time: Earth's orbit is elliptical and its axis is tilted, so the sun's apparent motion is not uniform. True solar time and mean solar time therefore swing back and forth across the year. Both parts together produce the true solar time of the birth moment.
For BaZi, the most direct effect is on the hour pillar. The day is divided into two-hour branches. Once the corrected time crosses a boundary, the hour pillar changes from one pair of characters to another. Near the zi hour, it can also bring the day-change convention into play. The effect does not always occur, but when it does, the entire hour pillar changes.
The year and month pillars are fixed by the Start of Spring and solar-term instants, which is a separate convention. True solar time acts mainly on the hour pillar and the zi-hour boundary and should not be used to explain every difference in the four pillars. Blending time correction, solar-term boundaries and the zi-hour day change into one issue is the most common source of misdiagnosis.
There is also a practical problem: tools do not handle this uniformly. Some enable it by default and some leave it off; some correct only for longitude and some include the equation of time; a birthplace given only at city level introduces its own error. 'True solar time' is therefore not a single universal answer but a convention that needs to be stated.
Comparison table: clock time vs true solar time
| Dimension | Clock time (e.g. Beijing time) | True solar time |
|---|---|---|
| Definition | The time used uniformly within a legal time zone | Local time corresponding to the sun's position at the birthplace |
| Corrections | None | Longitude difference and the equation of time |
| Effect on the hour pillar | Sorted directly into a two-hour branch | After correction, the moment may fall into a different branch |
| Effect on the day pillar | Handled by the zi-hour day-change rule | Once the moment is shifted, the zi-hour boundary may become more sensitive |
| Effect on the year and month pillars | Fixed by the Start of Spring and solar-term instants | Does not change solar-term instants and should not be used to explain year or month pillar differences |
| Largest difference | When the birthplace is far from the standard meridian | Near an hour boundary, and near the zi hour |
| Tool convention | Used by default in many products | Some default on, some default off, some correct for longitude only |
This table is about time conversion only; it does not rank which convention is more 'spiritually effective'. A convention can be chosen, but it must be stated. Without that, a user cannot trace the source of a difference.
How to verify: six steps to see whether true solar time changed your chart
Step one: confirm the birthplace, not just the province.
The longitude correction is calculated from the birthplace, and a city and a county town can differ. Give the city or county where possible, and check whether the tool matched the right coordinates. If the birthplace is wrong, the time correction that follows is meaningless.
Step two: confirm which kind of time was recorded.
The time on a birth certificate is usually local clock time, but daylight saving, a different time zone or a birth abroad can change the record. Clarify first whether the time is standard time, daylight saving time, or a family memory of 'roughly around then'. If the input layer is uncertain, no amount of precision in the correction can recover it.
Step three: check the direction of the longitude difference.
For a birthplace east of the standard meridian, local time usually reads later than clock time; west of it, earlier. Establish the direction first, then see whether the difference is large enough to cross an hour boundary. Get the direction wrong and the hour pillar moves the opposite way.
Step four: include the equation of time.
The equation of time varies between positive and negative across the year, so the longitude difference alone does not give a complete true solar time. The difference between tools often comes down to whether the equation of time is included. You do not have to calculate it by hand, but knowing it exists explains why two tools differ by a few minutes.
Step five: cast one chart with true solar time on and one with it off.
The birth information form allows true solar time to be switched on or off. Keep the date, time and birthplace fixed, change only this setting, and compare the four pillars. In many cases the hour pillar does not move; when it does, your birth moment was already close to an hour boundary.
Step six: read the boundary notice instead of guessing.
When true solar time moves the hour branch, the notice beside the chart shows the clock-time branch, the true-solar-time branch and the approximate offset. It explains the difference without choosing a convention for you. You can follow your own study habits in leaving it on or off, but keep the same setting when comparing readings of the same chart.
How we approach it
This site treats true solar time as a switchable, explainable convention rather than a hidden black box. The specific practice is:
Correct first, then cast. Once the birth information is matched to a city, the engine calculates true solar time from the birthplace's longitude and the equation of time, and only then decides the hour branch. Calculation and charting are separate steps, and the same input returns the same result.
Default and switch are both stated. True solar time can be switched on or off in the birth information. When a city is matched and its longitude is available, the default is on, and you can turn it off to compare. Whether it was enabled is recorded in the chart result, so you can see which convention produced it.
A notice appears when the branch changes. When true solar time would move the birth into another hour branch, the notice states the clock-time branch, the true-solar-time branch and the approximate offset in minutes. Charts that do not cross a branch do not show this notice, which avoids unnecessary noise.
The zi hour and solar terms are explained separately. The zi-hour day change is a school convention, while a solar-term boundary is a calendar boundary; neither substitutes for the other. The notices explain each category separately, and the reading does not present true solar time as the sole cause of every difference.
No promise of matching every tool. A different standard meridian, equation-of-time formula, birthplace precision or on/off setting can each produce a different result. We state our convention and acknowledge that another tool may use a different one. What can be compared is the convention, not an aura of mystery.
Limits and disclaimer
True solar time is a time-conversion convention, not a guarantee of greater accuracy. It may change the hour pillar or may change nothing at all; whether to use it depends on the convention you accept and on the reliability of the recorded birth time. BaZi readings do not provide medical diagnoses, disaster claims, investment advice or legal conclusions. For major decisions, rely on real-world conditions and qualified advice.
Frequently asked questions
Beijing time is already the national standard. Why calculate anything else?
What is unified is clock time, not the position of the sun. Birthplaces at different longitudes have different gaps between local solar time and Beijing time. The hour pillar is divided by local hours, so you need that gap to judge whether the hour assignment is stable.
What is the equation of time, and why does it change through the year?
The equation of time is the difference between true solar time and mean solar time. It comes from Earth's elliptical orbit and the tilt of its axis. It swings between positive and negative across the year, so the correction at one longitude differs by date.
If I was born near the top of the hour, should I turn true solar time on?
The clock minute itself is not the criterion; what matters is whether the correction crosses an hour boundary. Near a boundary, switching true solar time on or off can change the hour pillar; far from one, both settings usually give the same pillar. Casting once with each setting is the most direct test.
How is it calculated for someone born abroad?
First confirm whether the record uses local standard time, daylight saving time or Beijing time, then convert using the birthplace's time zone and longitude. Beijing time cannot simply be applied to a foreign birthplace; different time zones and standard meridians produce different corrections. If the input convention is unclear, resolve the record first.
Is true solar time more accurate or more traditional?
It is a time convention closer to the sun's position at the birthplace, which is not the same as a more accurate reading. Whether to adopt it depends on the charting tradition and consistency you choose. Comparisons of the same person over time should keep the same setting to remain comparable.
Why do some sites default it on and others off?
Because true solar time is an optional convention, not one that everyone accepts. A default reflects a product choice, not a verdict on correctness. When you see a difference, check whether both sides enabled true solar time, whether both include the equation of time, and whether birthplace precision is the same.
Can the zi hour and true solar time cause problems at the same time?
Yes. If a birth is near the zi hour and true solar time pushes the moment across the boundary, both the hour pillar and the day-change convention may be affected. Read the two notices separately: one concerns time correction, the other a school's rule for changing the day; neither replaces the other.
What is this site's default, and can I change it?
The birth information form provides a true solar time switch. When a city is matched and its longitude is brought in, the default is on, and it can be turned off manually. With it on, the engine converts using longitude and the equation of time; whether the branch changes is explained in the chart notice.
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Updated 2026-10-20