Free · no sign-up · minute-accurate
True solar time calculator for BaZi
Enter a birth date, time and city to see the true solar time correction applied to a BaZi chart: the longitude term, the equation of time, the net shift — and whether that shift moves the hour pillar. Clocks follow time-zone boundaries; the sun does not.
The short version
True solar time is clock time corrected so that noon means the sun is actually at its highest point. Two corrections do it: longitude (how far your birth city sits from the middle of its time zone, at 4 minutes per degree) and the equation of time (up to about ±16 minutes, because Earth’s orbit is elliptical and its axis is tilted). Total shift is typically under half an hour; it can reach two hours at the western edge of a very wide time zone, like China’s.
Our engine applies it by default and shows the correction on every result. You can switch it off — some schools do not use it — which is why the correction is printed rather than silently baked in.
True solar time for your birth data
Free · no sign-up
Pick your birth city for the full correction (longitude + equation of time). Without a city we can still show the equation of time, which is always applied.
How it’s calculated
- 1
Read the clock, then find the real instant
A wall-clock time plus an IANA time zone resolves to one exact instant. We use the zone’s full historical rules — including China’s 1986–1991 daylight saving and every other DST period — so a summer birth in the 1980s is not treated like a summer birth today.
- 2
Correct for longitude
Each time zone is 15° wide and its clock is set by the middle meridian. Four minutes of time for each degree away from it: a city 2° east of the meridian is 8 minutes ahead of its own clock time. This is the term that gets large — up to about ±2 hours for cities near the edges of wide zones.
- 3
Correct for the equation of time
Earth’s orbit is elliptical and its axis is tilted, so the true solar day varies through the year. The difference between mean and apparent solar time swings between roughly −14 minutes in mid-February and +16 minutes in early November. It is the same correction sundials need.
The full rule set — every convention, in order, with the engine field names — is published on the methodology page.
The two corrections, and which one actually matters
Almost every explanation of true solar time lumps the two corrections together. They behave very differently, and knowing which is which tells you immediately whether your chart is at risk.
The longitude term is geographic and permanent for a birth place. It is large exactly where time zones are wide or where a country uses one zone far wider than 15° — China (UTC+8 from roughly 73°E to 135°E), India, Spain, and Argentina are the usual offenders. For Urumqi, in China’s far west, the longitude term alone runs about −130 minutes: a morning birth recorded at 10:00 belongs to a 时辰 more than two hours earlier.
The equation of time is astronomical and permanent for a birth date. It is bounded by about ±16 minutes, so on its own it can move an hour pillar only if the birth time sits within a quarter-hour of a 时辰 boundary — which, for random births, happens to roughly one person in eight.
Why the hour pillar is the pillar at risk
The hour pillar is built from the 时辰 — one of twelve two-hour blocks, starting at 23:00 with 子. BaZi has no finer resolution than that, so a shift of a few minutes only matters if it carries the corrected time across a block boundary.
That gives a clean test, and this page runs it: compute the chart on the clock time, compute it again on the true solar time, and compare the two hour pillars. “Unchanged” is the common answer — which is useful to know, because it means the correction did not touch your chart.
The day and month pillars can in principle be affected too, but only when the correction crosses midnight or a solar-term instant — and both are rarer than the hour-pillar case, because the boundaries are fixed to the sun rather than to the clock.
What our engine does, and how to turn it off
True solar time is our default. The methodology page lists it as the default time basis for the engine, and every result prints the correction it applied — any summer time taken off the clock, the longitude term, the equation of time, and the net shift, in minutes.
It is off if you want it off. The calculator has a switch, and the chart is recomputed on your entered clock time with no correction. We take that position because schools genuinely disagree: the case for true solar time is astronomical and strong, but the case for following the civil clock is that the classical texts predate standard time zones entirely.
What we will not do is apply a correction without showing it. A tool that silently shifts your birth time by twenty minutes is asking you to trust a number you cannot see.
- Longitude correction is applied only when a birth city (or longitude) is supplied.
- Equation of time is applied whenever true solar time is on, even with no city.
- Historical daylight saving comes from the IANA zone database, not a fixed offset.
- The exact instant used for solar-term boundaries is always the astronomical one, with or without the correction.
Where the correction changes readings most
If you were born near the middle of your time zone, at a moderate latitude, and not within a few minutes of a 时辰 boundary, true solar time will not change your hour pillar. Most people are in that group.
The exceptions cluster: western China, western India, Spain, and anywhere on the western edge of a large zone; births recorded during a daylight-saving period; and birth times that fall within about fifteen minutes of an odd hour. The worked example below includes one of each kind.
A worked example
Four cases, all computed by this engine. The first shows a correction that changes nothing; the second crosses a 时辰 boundary by four minutes; the third is a birth in far-western China, where the same UTC+8 clock is more than two hours out of step with the sun; the fourth was recorded inside a daylight-saving window, where the clock itself had already been moved.
Input
- Date
- 4 Feb 1990
- Time
- 10:00
- Place
- Taipei, Taiwan
- Time zone
- Asia/Taipei
Nearest 节
Next: 立春, 4 Feb 1990 10:14Birth at 10:00 — 14 minutes before the year boundary.
Corrected, chart unchanged
Taipei · Asia/Taipei
- Longitude
- 121.57° → +6 min 16 s
- Equation of time
- −13 min 37 s
Hour pillar
Xīn Sì
The most common outcome. The correction is real — it is printed on every chart we produce — but it stays inside the same 时辰, so all four pillars are identical either way. Knowing this is the point: the correction did not touch your chart.
Four minutes from a boundary
Taipei · Asia/Taipei
- Longitude
- 121.57° → +6 min 16 s
- Equation of time
- −13 min 37 s
Hour pillar
Gēng Chén
Same day, same city, four minutes later on the clock — and a correction of under eight minutes is enough to cross the 09:00 boundary. The hour branch moves from 巳 to 辰, which changes the hour pillar and everything read from it. This is the case a clock-time calculator gets wrong without ever telling you.
The clock itself is two hours out
Urumqi · Asia/Shanghai
- Longitude
- 87.62° → −129 min 32 s
- Equation of time
- −14 s
Hour pillar
Jiǎ Chén
Urumqi runs on UTC+8, the same clock as Shanghai, though it sits 30° further west. The longitude term swamps the equation of time, and the corrected time lands more than two hours earlier — a different 时辰 and a different hour pillar. Wide single-zone countries are where true solar time stops being a footnote.
Summer time had already moved the clock
Berlin · Europe/Berlin
- Longitude
- 13.40° → −6 min 23 s
- Equation of time
- −4 min 03 s
- Summer time removed
- −60 min
Hour pillar
Gēng Chén
Germany was on summer time that day, so the recorded clock already stood an hour ahead of standard time. That hour is not a solar correction at all — it comes off first, and the engine lists it separately so you can see it. Berlin sits close to the 15°E meridian its standard time is set from, so the longitude term that remains is small: almost the whole shift is the clock. This is the common case in Europe, in most of the United States for eight months of the year, and in China in 1986–1991.
A correction is not automatically a different chart
The first case is the honest default: a correction of a few minutes, and the hour pillar does not move. The second case changes the hour pillar on a four-minute margin. The third changes it because the clock itself is two hours from solar noon. The fourth changes it because an hour of summer time had to come off before anything else was applied. Same engine, same rule, four different outcomes — which is why we print the correction instead of just applying it.
Engine v1.1.0 · every correction above is printed by the engine on the result, not estimated.
The twelve boundaries, in true solar time
The hour pillar does not use minutes or a clock — it uses these twelve two-hour blocks. A correction can only change the hour pillar by pushing the birth time across one of these lines, which is why a correction under fifteen minutes is usually harmless and a correction over two hours almost never is.
| Branch | Animal | True solar time | Element of the branch |
|---|---|---|---|
| 子 | Rat | 23:00–01:00 | 水 midwinter |
| 丑 | Ox | 01:00–03:00 | 土 late winter |
| 寅 | Tiger | 03:00–05:00 | 木 early spring |
| 卯 | Rabbit | 05:00–07:00 | 木 mid-spring |
| 辰 | Dragon | 07:00–09:00 | 土 late spring |
| 巳 | Snake | 09:00–11:00 | 火 early summer |
| 午 | Horse | 11:00–13:00 | 火 midsummer |
| 未 | Goat | 13:00–15:00 | 土 late summer |
| 申 | Monkey | 15:00–17:00 | 金 early autumn |
| 酉 | Rooster | 17:00–19:00 | 金 mid-autumn |
| 戌 | Dog | 19:00–21:00 | 土 late autumn |
| 亥 | Pig | 21:00–23:00 | 水 early winter |
The first block straddles midnight: 子 runs 23:00–01:00 in true solar time, and a birth in the 23:00–24:00 half keeps the current day’s day pillar — one of the conventions listed on the methodology page.
Questions
- Does BaZi require true solar time?
- Schools disagree, so there is no single correct answer. Astronomically, true solar time is what the classical hour divisions were built on — the 时辰 are defined by the sun’s position, not by an office clock. Practically, many contemporary practitioners use civil time. Our engine defaults to true solar time and lets you switch it off, and the correction is printed on every result either way.
- How large is the correction in practice?
- The equation of time is always within about ±16 minutes. The longitude term is 4 minutes per degree from the middle meridian of your time zone, so it depends where you were born: near-zero in the middle of a zone, and up to about two hours at the extreme edge of a wide one, such as far-western China inside UTC+8.
- I don’t know my birth city. Can I still use this?
- Yes, partially. With only a time zone we apply the equation of time (up to about ±16 minutes) and skip the longitude term, since that needs a birth place. Picking a city — or entering a longitude — adds the larger of the two corrections.
- Can the correction change the day or the month pillar?
- It can, but rarely. The day pillar is built from the true solar time relative to midnight, and the month pillar from the exact instant of a solar term, so the correction only matters if it carries the birth across one of those instants. The hour pillar is by far the most sensitive, because its boundaries are every two hours rather than every day or every month.
- Was there daylight saving when I was born?
- Possibly, and it is a real source of wrong charts. We resolve the zone offset from the IANA time-zone database, which carries each zone’s historical rules — China observed DST from 1986 to 1991, for example, and a birth in a US summer is usually recorded in daylight saving time. A fixed offset of “UTC+8” or “UTC−5” gets some of those charts wrong.
- Does the correction affect a BaZi reading much?
- Structurally, it either changes the hour pillar or it does not — and this page tells you which. When it does, the fourth pillar of the chart changes, which changes the hour-stem ten god, the branch interactions involving the hour branch, and anything read from the hour position. When it does not, the reading is identical and the correction is a footnote.
Next
The engine’s conventions
True solar time is listed as the default time basis, alongside every other rule the engine applies.
OpenHeavenly stems & branches
The 时辰 boundaries the hour pillar is built on, and the 60-cycle the pillars are read from.
OpenDay master calculator
The pillar that a true-solar-time correction is least likely to change.
OpenFull chart
Get your own four pillars
The calculator is free and needs no account. It shows all four pillars, hidden stems, ten gods, your element balance and ten-year luck cycles — with the engine version on the result.
BaziPattern provides AI-generated readings for entertainment and cultural exploration purposes only. Readings are not predictions, or medical, legal, or financial advice.