Method · 真太陽時 · birth-time correction
True Solar Time in BaZi
True solar time (真太陽時) is local apparent solar time: the time the sun shows at the birthplace. It is derived from clock time by correcting for the birthplace's longitude offset from the time-zone meridian, for daylight saving if it applied, and for the equation of time. In BaZi it matters only when the correction moves a birth across a two-hour branch boundary or across midnight; most charts do not change. A calculator should state whether it applies the correction, asks for corrected time, or leaves it to you. kaucim.ai asks you to choose the two-hour window yourself.
- Chinese
- 真太陽時 (zhēn tài yáng shí); also 真太阳时
- Also called
- Local apparent solar time, apparent solar time, sundial time
- Three corrections
- Longitude (4 minutes per degree from the zone meridian), daylight saving (usually one hour), equation of time (about −14 to +16 minutes through the year)
- Affects
- The hour pillar; occasionally the day pillar when the corrected time crosses midnight; the Zi Wei hour branch in the same way
- On kaucim.ai
- Not applied automatically. You choose the two-hour window; the free chart marks which parts depend on the hour
Why clock time is not solar time
BaZi's twelve hour branches were defined by the sun: Wu (午) is the two hours around local noon, Zi (子) the two hours around local midnight. Modern clock time is set to a zone meridian and, in many places, shifted an hour in summer. A birth recorded at 12:50 on the clock can sit at 12:13 by the sun, or at 13:40. Whether that matters depends on how close the clock time is to a branch boundary.
The three corrections
- Longitude
- Solar time runs four minutes earlier for every degree west of the zone's meridian and four minutes later for every degree east. This correction is fixed for a birthplace.
- Daylight saving
- If daylight saving was in force on the birth date, subtract one hour from the clock time first. Historical daylight-saving rules vary by country and year; Hong Kong used it until 1979, Taiwan until 1979, mainland China from 1986 to 1991.
- Equation of time
- The sun runs ahead of or behind mean time through the year because of the Earth's tilt and orbital shape. The offset is the same everywhere on a given date and ranges from about 14 minutes behind in mid-February to about 16 minutes ahead in early November.
| Birthplace | Longitude | Zone meridian | Correction |
|---|---|---|---|
| Hong Kong | 114.2° E | 120° E (UTC+8) | -23 min |
| Beijing | 116.4° E | 120° E (UTC+8) | -14 min |
| Singapore | 103.8° E | 120° E (UTC+8) | -65 min |
| Taipei | 121.6° E | 120° E (UTC+8) | +6 min |
| Tokyo | 139.7° E | 135° E (UTC+9) | +19 min |
| London | 0.1° W | 0° (UTC) | 0 min |
| New York | 74.0° W | 75° W (UTC−5) | +4 min |
| Los Angeles | 118.2° W | 120° W (UTC−8) | +7 min |
| Date | Correction |
|---|---|
| 1 January | -3 min |
| 11 February | -14 min |
| 15 March | -9 min |
| 15 April | 0 min |
| 14 May | +4 min |
| 13 June | 0 min |
| 26 July | -6 min |
| 1 September | 0 min |
| 3 November | +16 min |
| 25 December | 0 min |
True solar time = clock time − daylight saving + longitude correction + equation of time. The two tables give the last two terms; the daylight-saving term is one hour or nothing.
Worked example
A birth in Hong Kong on 11 February at 13:10 by the clock, no daylight saving. Longitude correction −23 minutes, equation of time −14 minutes: true solar time is 12:33. The clock time sits in Wei (13:00 to 15:00); the solar time sits in Wu (11:00 to 13:00). This birth should be entered with the Wu window.
The same clock time on 3 November gives −23 + 16 = −7 minutes, so 13:03, which stays in Wei. And a birth at 14:00 on either date stays in Wei whatever the correction. The correction only matters within roughly forty minutes of a boundary, which is why most charts do not change.
When to apply it, and when not to
- Apply it when the clock time is within about 40 minutes of an odd hour (01:00, 03:00, 05:00 and so on), because those are the branch boundaries, or within an hour of midnight, because midnight is also the day boundary.
- Apply it with care when the birth was under daylight saving or in the far west of a wide time zone (western China, Spain, parts of the United States), where the correction can exceed an hour.
- Do not bother when the clock time is near the middle of a two-hour window. A 14:00 birth is Wei in every reasonable correction.
- Do not apply it twice. If a hospital record or another calculator already gives solar time, enter that window directly.
How calculators handle it
| Approach | What the tool does | What to watch |
|---|---|---|
| Reader chooses the window (kaucim.ai) | Asks for a two-hour window and does not adjust it | Check the boundary cases yourself using the tables above |
| Automatic from birthplace | Asks for a city, applies longitude and sometimes the equation of time and daylight saving | Whether it applied all three, and whether its daylight-saving history is correct for the year |
| Corrected time as input | Expects you to enter solar time | Entering clock time here shifts every boundary birth the wrong way |
Two calculators that disagree on an hour pillar for a birth near a boundary usually differ here, not in the pillar arithmetic. The traditional remedy when the record is uncertain is to cast the chart both ways and read which one fits the life; the free BaZi chart marks the hour-dependent parts so the comparison is quick.
Common mistakes
- Applying the longitude correction with the wrong sign. East of the meridian the sun is ahead, so add; west, subtract.
- Forgetting historical daylight saving. A 1975 summer birth in Hong Kong was recorded under daylight saving; a 1995 one was not.
- Correcting a birth that is nowhere near a boundary and then worrying about the minute. The chart resolves to two-hour windows.
- Treating the Zi hour as one window. On kaucim.ai it is two: early Zi 00:00 to 01:00 on the new date, late Zi 23:00 to 24:00 on the old date, and the corrected time decides which.
Questions people ask
Does kaucim.ai use true solar time?
Not automatically. The calculator asks you to choose the two-hour window, so you can apply the correction yourself when your clock time sits near a boundary. The hour-dependent parts of the free chart are marked.
How much can the correction be?
Up to about an hour and a half in extreme cases: one hour of daylight saving, up to an hour of longitude in a wide zone, and up to sixteen minutes of equation of time. In Hong Kong without daylight saving the total stays within about 40 minutes.
Does true solar time affect Zi Wei Dou Shu too?
Yes, in the same way. Zi Wei uses the same two-hour branch for the birth hour, so a correction that moves the branch moves the Life Palace and every star.
I do not know my birthplace's daylight-saving history. What then?
Look it up for the year and country; most national time-zone histories are documented. If the record is genuinely uncertain and the clock time is near a boundary, cast both windows and compare.
Related concepts
Sources and calculation version
- Equation of time · the annual curve the sample dates are read from
- BaZi calculation rules on kaucim.ai · the rules table on the calculator page
Last checked against the chart engine on 2026-09-14.
Work out the window from the tables above, then cast the free chart. If two windows are possible, cast both and compare the marked parts.
Cast the chart with your corrected window