A term explained
真太陽時 (true solar time): why this does not use your phone's clock
Short version: true solar time is the time you get from where the sun actually is, and it is not the time on your phone. 奇門遁甲 reckons the 時辰 — the two-hour branch — from the first one.
How far apart are they? In London, anywhere from 15 minutes behind to 16 minutes ahead, depending on the date. That sounds like nothing, but the branches are two hours long with boundaries on the odd hours, so a quarter of an hour is enough to put you in the next branch — and a different branch is a different chart.
Difference one: a clock is shared by a whole time zone. The sun is not.
Every time zone has a standard meridian, and a clock there tells the same time from one edge of the zone to the other. The sun does not oblige. What is left over is fixed for any given place, and it is purely a matter of longitude:
| London | 0.13°W — half a minute. London sits almost exactly on its own meridian, which is not a coincidence: it defined it. |
| Tokyo | 139.69°E against a 135°E meridian — the sun runs 19 minutes early. |
| Singapore | 103.82°E on a UTC+8 clock — the sun runs 65 minutes late. |
| Madrid | 3.7°W on a UTC+1 clock — 75 minutes late in winter, and 135 on summer time. |
| Kashgar | 75.99°E, and all of China is UTC+8 — 176 minutes late, nearly three hours. |
Madrid is the one worth sitting with. Spain is west of Greenwich and keeps central European time, so on a summer afternoon the clock is more than two hours ahead of the sun. It is also why Spanish dinner looks late to everyone else and does not feel late in Madrid.
Difference two: the sun itself runs fast and slow (the equation of time)
The earth's orbit is an ellipse and its axis is tilted, so the sun does not keep even time. Over a year it runs up to about 16 minutes ahead of an imaginary average sun (early November) and about 14 minutes behind it (early February). That gap has a name: the equation of time.
This part is not estimated. It comes from the Swiss Ephemeris — the same source as the solar-term boundaries.
Add them up and your clock drifts half an hour across the year
The longitude part is fixed for your city; the seasonal part is the same for everybody. Put together, one city at one clock time moves like this:
| 11 Feb | London, 2:00 pm → true solar 1:45 pm, 15 minutes behind |
| 3 Nov | London, 2:00 pm → true solar 2:16 pm, 16 minutes ahead |
Half an hour between the two dates — and that 30-minute swing is the same number in Taipei, Tokyo, Seoul and Madrid, because it is the sun doing it, not the place. Usually it changes nothing. The trouble is that the branch boundaries sit on fixed hours, so if your question lands near one, this pushes you over it.
Pushed over, it is not a near miss — it is a different chart
Measured by running this app's own engine. 3 November 2026, in London, clock reading 10:45 am:
| ✗ | Clock only — 巳 branch, the hour lands in palace 3 |
| ✓ | True solar time (11:00) — 午 branch, the hour lands in palace 2 |
That palace is where a reading starts: the square you look at first for the thing you asked about. Moving it from 3 to 2 moves the centre of gravity of the whole chart.
And on a February morning the same 10:45 gives the same answer either way — nothing changes. So this is not "always a big difference"; it depends on the date. Sometimes it is irrelevant and sometimes it is a different chart, which is exactly why it cannot be skipped.
What this app does: the place you type is converted to true solar time automatically, and the reading screen states how many minutes it corrected by, so you can see it. A place we cannot resolve is cast from the clock instead — no quiet guessing.排盤: how the chart is worked out
Where a lot of writing on this gets it backwards: solar terms must NOT be corrected
A solar term (立春, 秋分 and the rest) is not a day. It is one instant: the moment the earth reaches a particular angle. That instant is the same for the entire planet and has nothing to do with where you are standing — like the time of an eclipse, which is not different in Taipei and Madrid.
So correcting the clock to solar time *before* looking up the solar term makes the boundary slide with your longitude. We measured it on 28 September 2026: done that way, roughly 204 minutes a year get labelled with the wrong term in Taipei — and inside those minutes the 陰遁/陽遁 direction, the 局 number and the whole plate come out different.
- Solar terms: read at the true instant (get your time zone right and that is all), never longitude-corrected.
- The 時辰, the day pillar, the 符頭: true solar time, corrected.
That is how the app splits it now, and there is a test guarding this one line specifically, so it cannot quietly get merged back.
The honest limits: if the place cannot be resolved there is no correction at all — identical to leaving the place blank. And if your device can only offer raw coordinates with no name, the time zone is inferred from longitude and can be an hour out. In both cases we would rather not guess.
If a few minutes can change which branch you are in — then how does picking a good time work at all?擇吉: so when is a better time to move
Makes sense — but you would rather not cast one yourself?
These three steps were always a fixed set of rules; they just used to run in a human head and pass from master to student. We hand the arithmetic to a computer and the explaining to AI. All you do is write down what you are weighing.
Write your question奇門遁甲 is a traditional Chinese practice for choosing timing. This page is background reading — not medical, legal or financial advice, and it will not tell you how anything is going to turn out.