True Solar Time in BaZi

The twelve BaZi hours were defined by the sun: the 午 hour is centred on local noon, when the sun is highest, and the 子 hour on local midnight. Modern clocks, by contrast, use one time for a whole time zone. True solar time (真太阳时) converts the clock time on a birth certificate back to the sun's actual position at the place of birth. This article explains the calculation, when it matters and how this calculator handles it.

Two corrections

True solar time differs from clock time for two independent reasons.

1. Longitude. A time zone uses the clock time of a reference meridian — 120°E for China Standard Time, 0° for Greenwich, 15°E for Central European Time. The sun crosses each degree of longitude in four minutes, so a place west of its zone meridian sees the sun later than the clock suggests. Beijing (116.4°E) is about 14 minutes behind 120°E; Chengdu (104.1°E) about 64 minutes; Urumqi (87.6°E) about 130 minutes.

2. The equation of time. Even at the zone meridian, the sun is not exactly overhead at 12:00 every day. Because Earth's orbit is elliptical and its axis is tilted, apparent solar time runs up to about 16 minutes ahead (early November) or 14 minutes behind (mid-February) mean time. This variation is called the equation of time.

True solar time = clock time − daylight saving (if any) + longitude correction + equation of time.

A worked example

Suppose someone was born at 07:50 clock time on 10 February in Chengdu.

• Longitude correction: (104.07 − 120) × 4 min ≈ −64 minutes.
• Equation of time around 10 February: about −14 minutes.
• True solar time ≈ 07:50 − 64 − 14 = 06:32.

07:50 by the clock falls in the 辰 hour (07:00–09:00); 06:32 true solar time falls in the 卯 hour (05:00–07:00). The hour pillar changes. A birth at 08:30 clock time, on the other hand, gives about 07:12 true solar time — still 辰 — and the pillar does not change.

Daylight saving and historical time zones

Before any solar correction, the clock time must be converted to standard time. Two historical issues cause most errors:

• Daylight saving time. China used daylight saving from 1986 to 1991; Hong Kong, Taiwan, most of Europe and North America have used it in many years. A birth recorded during DST is one hour ahead of standard time.
• Changing zones. Before 1949 China used several time zones; many countries have changed their standard offset over the twentieth century.

This calculator applies the birth city's historical time-zone rules for the exact date from the IANA time-zone database, which covers daylight saving and most changes of standard offset, so you normally enter the clock time as recorded. Where two clocks are in everyday use — for example in Xinjiang, where both Beijing time and local Xinjiang time are used — check which clock the birth was recorded in. When a clock time is ambiguous (repeated in autumn) or did not exist (skipped in spring), the calculator flags it instead of guessing.

When it matters

True solar time only changes a chart when the corrected time crosses a boundary:

• Hour boundaries fall on odd hours: 01:00, 03:00, 05:00 … 23:00. If your corrected time is near one, the hour pillar may change.
• The day boundary at 23:00 (or midnight, depending on the Zi-hour setting) can change the day pillar — and therefore the Day Master.
• Solar-term boundaries use absolute instants and are not affected by solar time, but they are affected by correct time-zone handling.

If your birth time is in the middle of a two-hour period, true solar time and clock time almost always give the same chart.

Should you use it?

Many practitioners use true solar time because the hours were defined by the sun. Others use clock time, arguing that the convention of the day is what matters, or that their teacher did so. Both positions have long histories. This calculator defaults to true solar time and offers a switch in Advanced settings. Whichever you choose, use it consistently across all the charts you compare. See true solar time vs clock time.

Typical corrections by city

Add the equation of time for the date (between about −14 and +16 minutes) and remove daylight saving if it was in force.

Common mistakes

• Using the current city instead of the birth city. Longitude is about where the birth happened.
• Forgetting daylight saving. A one-hour error can change the hour pillar.
• Applying solar time twice. If a birth time was already recorded in local mean time (some old records were), do not correct it again.
• Over-correcting rounded times. Hospital records are often rounded to five or fifteen minutes; if the corrected time is near a boundary, check both candidate hours.

Key points

• The hour pillar was originally defined by the sun's position, not by a clock, so true solar time is the traditional basis for it.
• The correction has three steps: historical time zone and daylight saving, then longitude (four minutes per degree), then the equation of time (about −14 to +16 minutes).
• Corrections are largest far from a time zone's reference meridian, such as western China on Beijing time or Spain on Central European Time.
• Births within about ten minutes of an odd hour boundary deserve a check of both candidate hours.
• True solar time can move a birth across 23:00 and so change the day pillar under the default convention.
• Solar terms are fixed instants. True solar time does not move them, though clock errors can matter near a term.
• Most differences between online calculators come from three settings: true solar time, city and time zone, and daylight saving.
• The calculator shows each correction step, so you can see where the hour pillar comes from.

Key terms in this article

True Solar Time · 真太阳时

True solar time is clock time corrected for longitude (four minutes per degree from the zone meridian) and the equation of time (up to about ±16 minutes over the year). It places the sun at its actual position, which is what traditional hours follow.

Hour Pillar · 时柱

The hour pillar covers one of twelve two-hour periods (时辰), starting with 子 at 23:00–01:00. Its stem is derived from the day stem by the "five rats" rule. Because the boundary is solar, true solar time can move a birth into the neighbouring hour.

Zi Hour (Rat Hour) · 子时

The 子 hour runs 23:00–01:00. Schools disagree on whether the day changes at 23:00 (early-rat convention, 早子时 counted to the next day) or at 00:00 (late-rat convention). The choice changes the day pillar for births between 23:00 and midnight.

Solar Terms · 二十四节气

The twenty-four solar terms divide the sun's path into 15° steps. The twelve 节 (jie) terms such as 立春 and 惊蛰 begin BaZi months; the twelve 中气 terms such as 雨水 mark mid-months. Each term has a precise astronomical moment, not just a date.

How the correction is calculated

The calculator works in three steps. First it converts the clock time to Universal Time using the historical rules for the birth place, including any daylight saving in force on that date. Second it adds four minutes for every degree of longitude east of Greenwich (or subtracts for west), giving local mean time. Third it adds the equation of time for that date, which ranges from about −14 minutes in February to about +16 minutes in early November, giving apparent solar time. The result is the time the sun actually indicated at the birth place.

Each of these steps is ordinary astronomy. The traditional choice is to use the result to determine the hour branch, because the twelve hours were originally defined by the sun's position, not by a clock.

Edge cases worth checking

• Births near an hour boundary. If the corrected time lands within ten minutes of an odd hour (01:00, 03:00 …), note both candidate hour pillars.
• Births near midnight. True solar time can move a birth across 23:00, which changes the day pillar under the default convention. See the Zi hour and the day boundary.
• Births near a solar term. Solar terms are fixed astronomical instants, so true solar time does not move them; but a clock error of an hour can, if the term falls on the same day.
• Uncertain records. Hospital records are often rounded to five or fifteen minutes. If the correction is small compared to that rounding, it changes little.

Comparing calculators

If another calculator shows a different hour pillar, check three settings first: whether it uses true solar time, which city and time zone it assumed, and whether it applied daylight saving. These three differences account for most disagreements between online charts. The methodology page lists the exact rules used here.

Frequently asked questions

Does true solar time change my year or month pillar? No. Those depend on solar-term instants, which are absolute. Only correct time-zone handling matters there.

I only know the province, not the city. Choose the main city; if the corrected time is near a boundary, check both hours.

Is local mean time the same as true solar time? No. Local mean time applies only the longitude correction; true solar time also applies the equation of time.

Next: what is BaZi?, or the true solar time calculator.