Two people born on the same date, at the same time on the clock, can end up with two different charts — and neither is a mistake. The clock on the hospital wall said 12:00. The sun had a different opinion.
True solar time (真太阳时) is the sun's own time at your exact spot on Earth. It is not a variation or a school of thought. It is arithmetic — and in Zi Wei Dou Shu and BaZi it is the difference between reading the right hour and the wrong one.
| Question | Answer |
|---|---|
| What is true solar time? | The time read off an actual sundial at your location: local noon is the moment the sun crosses your meridian |
| How is it different from clock time? | Three corrections — longitude, the equation of time, and legal clock changes (DST / historical zones) |
| How big can the gap be? | Up to roughly two hours from longitude alone inside a single time zone — over three hours in the far west during DST years; ±16 minutes from the equation of time |
| Does it change the chart? | Yes — the birth hour sets the Life Palace and Body Palace, and near midnight it can move the day. The hour pillar moves with it |
The Earth turns 15° every hour, so one degree of longitude is worth four minutes: offset = (your longitude − zone meridian) × 4 minutes. West of the meridian your clock runs ahead of the sun; east of it, behind.
| City | Longitude | Standard offset | Meridian compared against | Correction |
|---|---|---|---|---|
| Urumqi | 87.62°E | UTC+8 | 120°E | −129.5 min |
| Chengdu | 104.07°E | UTC+8 | 120°E | −63.7 min |
| Beijing | 116.40°E | UTC+8 | 120°E | −14.4 min |
| Shanghai | 121.47°E | UTC+8 | 120°E | +5.9 min |
| Tokyo | 139.69°E | UTC+9 | 135°E | +18.8 min |
| New York | 74.00°W | UTC−5 | 75°W | +4.0 min |
Neither direction is "wrong" — the clock was never built to measure your local sky.
Even standing exactly on the zone meridian, your clock would still drift away from the sun through the year. The orbit is an ellipse and the axis is tilted; the two effects add up to a wandering offset:
| Date | Equation of time | The sun is… |
|---|---|---|
| 11 February | −14 min 15 s | behind the clock |
| 15 April | 0 | in agreement |
| 14 May | +3 min 41 s | ahead of the clock |
| 26 July | −6 min 30 s | behind the clock |
| 3 November | +16 min 25 s | ahead of the clock |
This value is the same everywhere on Earth on a given date — it is astronomy, not geography. It never exceeds about 16 minutes in either direction, which makes it look negligible next to the longitude term. It is not: fifteen minutes is enough to cross an hour boundary on the wrong day.
The third correction has nothing to do with the sun. It comes from laws.
| Region | Period | Clock vs the sun |
|---|---|---|
| China | before 1949 / 1949 onward | Regional zone systems gave way to one zone (UTC+8) nationwide — the biggest single cause of large offsets today |
| China | 1986–1991 | Daylight saving in summer, +1 hour on the clock |
| United States | 1918 onward, current rule | Second Sunday in March to first Sunday in November, +1 hour |
| Europe / UK | 1916 onward | Late March to late October, +1 hour |
A birth at 12:00 on 1 July 1988 in Beijing resolves to a true solar time of 10:41; the same clock time on 1 July 1992, after DST was abolished, resolves to 11:41. One hour of difference from the same clock reading, and no arithmetic error anywhere. That is why memory is a poor source for a birth hour: families remember the number the clock showed, and clocks have been legislated.
Zi Wei Dou Shu divides the day into twelve two-hour blocks, the shíchen (时辰) — all times here are true solar time:
子 23:00–00:59 · 丑 01:00–02:59 · 寅 03:00–04:59 · 卯 05:00–06:59 · 辰 07:00–08:59 · 巳 09:00–10:59
午 11:00–12:59 · 未 13:00–14:59 · 申 15:00–16:59 · 酉 17:00–18:59 · 戌 19:00–20:59 · 亥 21:00–22:59
The Life Palace steps back one palace per hour block from the month position; the Body Palace steps forward one. One block of error is one palace of error — in both anchors at once — and the BaZi hour pillar moves with it. Live calculations, on the same engine the site runs:
| Case | Clock | True solar | Block | Life Palace | Body Palace | Hour pillar |
|---|---|---|---|---|---|---|
| Beijing, 15 June 1992 | 15:02 | 14:47 | 未 | 亥 | 丑 | 丁未 |
| Beijing, 15 June 1992 | 15:20 | 15:05 | 申 | 戌 | 寅 | 戊申 |
| Urumqi, 15 June 1992 | 12:00 | 09:50 | 巳 | 丑 | 亥 | 乙巳 |
Same date, and two of them the same clock time — eighteen minutes apart in the first pair, two thousand kilometres apart in the last — and three different charts. Only one set of inputs per person is correct, and you cannot tell which by reading the chart afterwards.
The 子 block (23:00–00:59) straddles midnight, and two conventions exist for births at 23:00–23:59 — day ending or day beginning. The choice moves the lunar day number, which is the input for the position of the Zi Wei star itself. The correction sharpens it: a birth at 00:40 in a city 100 minutes west of its meridian is, in solar terms, about 23:00 the previous evening — a different day and a different hour block.
You are not stuck — you are working with a smaller chart. Narrow the hour using anchors ("just before the morning news" beats "early morning"), test the two or three candidates, and then read the parts that do not depend on the hour: the year-based transformations and the star set fixed by year and month hold across all of them. Say so in the reading, too — "hour unknown" is an honest input, not the same as "hour guessed".
Birth time is resolved before anything else is calculated:
The correction is applied once, at the input stage — but it does not stay there. It propagates into four different layers of the chart, which is why a birth a few minutes from a boundary deserves more care than a birth in the middle of a block:
The point is not that clock time is untrustworthy. It is that the hour block is an input to almost everything, so a correction that looks small in minutes can be large in consequences. If your birth sits within twenty minutes of a block edge, treat both candidates as live questions rather than picking one and moving on.
If you want to see exactly what each of those layers does with the hour, the anchor step is on the Life Palace calculation, the star rules are on star placement formulas, and the full five-step sequence is on how a chart is calculated.
How large can the total correction be? Up to about two hours of longitude offset inside one time zone — Urumqi sits 129.5 minutes west of the 120°E meridian its clock is set to — plus ±16 minutes of equation of time, plus a possible 60 minutes of DST. For a birth in far-western China during a DST year, that is over three hours of total correction.
Do I need to bother if I was born around noon? Yes. Noon is not protected; what matters is the distance to an hour boundary. A noon birth in a city with a 65-minute offset lands at 10:55 in solar terms — near a block edge.
Does this affect the BaZi reading too? Yes — the hour pillar comes from the hour, and near midnight the day pillar is in play as well.
Can I just use the time on my birth certificate? Use it as the starting point: it records what the clock said. It is not yet the time the chart needs.
Take the birth time you have and the city you were born in — that is the whole test.
Generate your chart free at askziwei.com: city, date, and time are enough, and the result line shows the lunar date and the resolved true solar time before the palaces are drawn. If your hour sits within twenty minutes of a boundary, run it twice and compare the Life Palace.
Related reading: How a Chart Is Calculated · The Life Palace Calculation · Star Placement Formulas · The 12 Palaces · Life Palace · How to Read a Chart · Which App Is Accurate?
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