True Solar Time in BaZi: Why 17:30 in Ürümqi Is Not the You Hour
Wall-clock time is a political standard, not the sun's position. This page shows the two corrections that turn a clock reading into true solar time, with a worked example from western China where the gap reaches nearly two hours.
Direct answer
BaZi hour pillars are set by true solar time — the sun's actual position at the birth place — not by the wall clock. Converting a clock reading takes two corrections: a longitude correction of four minutes per degree away from the standard meridian (120°E for China), and the equation of time, which swings between roughly −14 and +16 minutes across the year. In Ürümqi (87.62°E) the combined correction on 1 December is about −119 minutes, which moves a 17:30 clock birth from the You hour (酉) back into the Shen hour (申).
Key terms in this page
- True solar time (真太陽時)(zhēn tài yáng shí)
- — time measured by the sun's actual position at the birth location; the basis for the hour pillar.
- Longitude correction (經度校正)(jīng dù jiào zhèng)
- — the offset between the birth longitude and the standard meridian, at four minutes per degree.
- Equation of time (均時差)(jūn shí chā)
- — the seasonal difference between apparent and mean solar time, from about −14 to +16 minutes.
- Standard meridian (標準經線)(biāo zhǔn jīng xiàn)
- — the meridian defining a time zone's clock; 120°E for China Standard Time.
- Hour pillar (時柱)(shí zhù)
- — the stem-branch pair for the two-hour period of birth; the pillar most sensitive to time corrections.
01What the clock hides: two corrections, not one
A clock in China shows UTC+8 everywhere from the Pamirs to the East China Sea, but the sun does not. The first correction is geometric: every degree east or west of 120°E shifts local solar noon by four minutes, so a birth in Ürümqi (32.38° west of the standard meridian) happens at a solar time about 129 minutes earlier than the clock says. The second correction is astronomical: Earth's orbit is elliptical and its axis tilted, so apparent solar noon drifts against mean solar noon over the year, adding or subtracting up to about sixteen minutes.
The two corrections are additive and can partially cancel. They matter most near hour boundaries: a correction of even a few minutes decides whether a birth belongs to the current two-hour period or the next one. This is why the hour pillar is the pillar most often wrong in automated charts, and why a birthplace is not optional data.
02Worked example and near miss: the same clock time in two cities
Take a birth recorded at 17:30 on 1 December 1990. In Beijing (116.4°E), the longitude correction is about −14.4 minutes and the equation of time on that date is about +10.6 minutes, so true solar time is roughly 17:26 — still inside the You hour (17:00–18:59), pillar 乙酉. In Ürümqi (87.62°E), the longitude correction is about −129.5 minutes and the same equation-of-time value applies, giving true solar time of about 15:31 — inside the Shen hour (15:00–16:59), pillar 甲申.
The near miss is instructive: the two births share the same clock reading and the same civil date, but differ by one hour pillar because of geography alone. Conversely, a birth at 15:01 in Ürümqi and one at 17:01 in Beijing both land in the Shen hour, despite looking two hours apart on paper.
03School variants: what software actually does
The dominant convention in professional practice, and the one used here, applies both corrections and sets the hour pillar by true solar time. Many consumer calculators skip them and use the wall clock directly; their charts agree with the corrected ones for most of the day and diverge near hour boundaries, especially at the western and eastern edges of a time zone.
A third variant applies the longitude correction but not the equation of time. That is a reasonable approximation in the middle of a season, but it can differ by up to sixteen minutes near the February and November extremes of the equation of time — enough to cross an hour boundary.
The standard meridian itself is worth stating: for China it is 120°E, for Korea and Japan 135°E. Using the wrong meridian shifts every correction by a fixed amount and produces systematically early or late hour pillars.
04Limits: what the correction cannot fix
True solar time depends on the birth location, so a chart without a birthplace can only approximate. The site's own convention is to state the assumption when the location is missing rather than silently defaulting to the provincial capital.
The correction does not resolve an uncertain birth time: if the record says “around five”, the honest output is two candidate hour pillars, not a confident single answer.
The correction also does not decide the day pillar. Near midnight, a large correction can push a birth across the 23:00 or 00:00 boundary, and that interaction must be handled by the day-rollover convention described on the late-Zi-hour page.
05Checklist
- The birthplace's longitude (not just the city name) was recorded
- The standard meridian for the time zone was identified
- The longitude correction was computed at four minutes per degree
- The equation of time for the birth date was applied
- The corrected time was compared against both adjacent hour boundaries
Frequently Asked Questions
Sources reviewed on 2026-10-31
- lunar-javascript 1.7.7 — wall-clock reference outputs used for the two-city comparison; verified 2026-10-31
- Bazi convention baseline v1 (conventions-v1.json) — Ürümqi case with −119 min applied correction, archived 2026-10-31
- Our BaZi engine (divination-core) true-solar-time handling — applies longitude correction plus astronomical equation of time
Computed values on this page were produced by running the engine versions listed above at the reviewed date; school disagreements are reported as disagreements, not resolved to a single answer.
Related entries
- Birth Time, Time Zones and True Solar Time: Get the Time Right FirstBirthplace, time zones, daylight saving, hour boundaries and the true solar time dispute — and why casting a chart should be separate from AI interpretation.
- The Month Pillar Changes at the Exact Solar-Term Minute — Not on the DateThe month pillar flips at an astronomical instant, which can fall in the middle of a double-hour. This page shows a birth at 21:58 charted with one month pillar and a birth two minutes later with the next — and why some software disagrees.
- Why Two BaZi Calculators Disagree: The Six Conventions Behind Different ChartsWhen two apps show different four pillars for the same birth data, the cause is almost always one of six stated conventions — not a calculation error. This page lists them, shows a side-by-side example matrix, and gives a four-step way to reconcile any two charts.
- Late Zi Hour (23:00–23:59): Does the Day Pillar Change at 23:00 or at Midnight?The late Zi hour is the one boundary where two BaZi traditions disagree about the day pillar itself. This page states both conventions, shows what each one outputs for the same birth time, and explains how to tell which convention your charting software uses.
Updated 2026-10-31