True Solar Time in BaZi
真太阳时 · 钟表在撒谎，太阳没有
Yes. The twelve two-hour blocks of a BaZi day were defined by the sun, not the clock. Casting a chart means converting clock time to true solar time (真太阳时) — daylight saving, longitude, equation of time — which can change your hour pillar.
Why does the clock differ from the sun?
Time zones are a railway-era convention: whole regions agree to pretend they share one noon. The earth turns 15° of longitude per hour, so every degree you stand west of your zone’s reference meridian puts the sun 4 minutes behind the clock. On top of that, the earth’s tilted axis and elliptical orbit make the sun run fast or slow through the year — the equation of time, which swings between about −15 and +16 minutes. And if daylight saving was in force, the clock was moved a further hour off the sun on purpose.
True solar time = clock time − daylight saving − distance from the meridian + equation of time. The three parts always add up exactly to the total correction, so the arithmetic can be checked line by line.
How big is the correction in your city?
Computed with the same solar-time module that powers the Auspice calculator (IANA historical time zones, NOAA equation-of-time approximation), for noon on 1 January and 1 July 2000. The two columns differ where daylight saving applies — note Sydney and São Paulo flip seasons.
| City | Longitude | Longitude part (fixed) | Total, Jan 1 | Total, Jul 1 |
|---|---|---|---|---|
| Vancouver | 123.1°W | -12 min | -16 min | -76 min |
| Los Angeles | 118.2°W | +7 min | +3 min | -57 min |
| Toronto | 79.4°W | -17 min | -21 min | -81 min |
| New York | 74.0°W | +4 min | 0 min | -60 min |
| São Paulo | 46.6°W | -6 min | -70 min | -10 min |
| London | 0.1°W | 0 min | -4 min | -64 min |
| Berlin | 13.4°E | -6 min | -10 min | -70 min |
| Dubai | 55.3°E | -19 min | -23 min | -23 min |
| Ürümqi (Beijing clock) | 87.6°E | -129 min | -133 min | -133 min |
| Singapore | 103.8°E | -64 min | -68 min | -68 min |
| Beijing | 116.4°E | -14 min | -18 min | -18 min |
| Sydney | 151.2°E | +5 min | -59 min | +1 min |
Minutes to add to clock time. The remainder between the longitude part and the total is the equation of time plus, where active, daylight saving (−60).
Two worked examples. A 2:30 pm birth in Vancouver on 15 July 1990 is 13:11 by the sun (-78 min: -60 daylight saving, -12 longitude, -6 equation of time) — enough to leave the Goat hour for the Horse hour. And an 8:00 am birth in Ürümqi the same day is 4:44 by the sun (-195 min), because the recorded clock was Beijing time — and 1990 China was under its brief daylight-saving experiment.
When does the correction actually change a chart?
Honestly: for most births, it doesn’t. The two-hour blocks flip at odd clock hours (1, 3, 5…), and a correction only matters if it walks the time across one of those boundaries. Our calculator flags an hour pillar as uncertain when the corrected time lands within 8 minutes of a boundary — in that case, read both charts rather than trust either. The people most likely to cross a boundary: births under daylight saving, births far from the time-zone meridian (western China, most of Spain, parts of Canada and Brazil), and recorded times that were rounded to the hour.
Which daylight-saving rules does the chart use?
Historical ones, not today’s. Daylight-saving rules change constantly, and a chart must use the rule in force on the birth date: mainland China observed DST from 1986 to 1991, Taiwan and Hong Kong had early periods of it, São Paulo kept it until 2019. We resolve birth times against the IANA time-zone database (via ICU), which encodes this history — the Ürümqi example above only comes out right because 1990’s rule differs from 2026’s.
What about births between 11 pm and midnight?
The Zi hour (子时) runs 23:00–01:00, straddling midnight, so the 23:00–24:00 slice must be assigned to a day by convention. We use the same-day convention (sect 2): your day pillar stays with the date you were born on. Another long-standing school rolls it to the next day, and no arithmetic settles the dispute — a birth in that hour genuinely has two defensible charts. The full convention list is on the method page.
What if the birth time is unknown?
Three of the four pillars come from the date alone, so an unknown hour costs you the hour pillar and nothing else — the day pillar, the one that means “you,” is safe. The Auspice calculatorcasts unknown-hour charts from what the twelve candidate hours share, and treats a remembered “around dinnertime” as the window it really is.
Every number on this page is reproducible: the exact weights and conventions are documented on the method page, and your own chart shows its correction broken into the same three parts — free, no account. For the wider system, read what BaZi is. Auspice is for reflection, not prediction.