Method · 真太陽時 · birth-time correction
True Solar Time in BaZi
True solar time (真太陽時) follows the sun at the birthplace. It adjusts clock time for longitude, daylight saving when applicable, and the equation of time. A correction can change a BaZi hour branch or move the birth across midnight. The English form on kaucim.ai offers clock time or an apparent-solar-time estimate for 42 listed US cities, Singapore, five Malaysian cities and the eight Australian capitals. Review the resulting date and hour branch before submitting. Other birthplaces use a manually chosen branch.
- 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
- Optional in the English form for 42 listed US cities, Singapore, five Malaysian cities and the eight Australian capitals. Enter the original clock time, choose a basis, then review the adjusted date and hour before submitting
Why clock time is not solar time
If you are checking your first chart, start with the BaZi guide and its worked Four Pillars example, then use the time corrections here to check births near a boundary.
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 |
| Sydney | 151.2° E | 150° E (UTC+10) | +5 min |
| Melbourne | 145.0° E | 150° E (UTC+10) | -20 min |
| Brisbane | 153.0° E | 150° E (UTC+10) | +12 min |
| Adelaide | 138.6° E | 142.5° E (UTC+9:30) | -16 min |
| Perth | 115.9° E | 120° E (UTC+8) | -17 min |
| Auckland | 174.8° E | 180° E (UTC+12) | -21 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. Those Hong Kong examples do not set a universal limit. In Singapore and western Malaysia, the longitude correction alone can exceed an hour.
Born in Australia
Australia runs three standard zones, and daylight saving is decided state by state. New South Wales, Victoria, South Australia, Tasmania and the ACT move their clocks forward one hour from the first Sunday in October to the first Sunday in April, a rule they have shared since 2008. Before 2008 the start and end dates varied by state and year. Queensland, Western Australia and the Northern Territory stay on standard time. Queensland used daylight saving in 1971 to 1972 and for three summers from 1989, then voted it down in 1992. Western Australia trialled it four times, most recently from December 2006 to March 2009, and voted it down in 2009.
On kaucim.ai, pick the capital city of the birth and enter the clock time from the record. In clock mode the form keeps that time and says when daylight saving was in force. In solar mode it reads the state's history for that year, removes the hour, then applies longitude and the equation of time.
A Sydney birth on 15 February 2000 at 13:40 by the clock falls in Wei. Daylight saving was in force, so standard time was 12:40. Sydney lies about 5 minutes east of its zone meridian and the equation of time that day is about minus 14 minutes, which puts solar time at about 12:30, in Wu. The same clock time in Melbourne comes to about 12:05, because Melbourne lies about 20 minutes west of the meridian. In Brisbane, which had no daylight saving that year, it comes to about 13:37 and stays in Wei.
When to apply it, and when not to
- Check the full correction before deciding whether it crosses an hour-branch boundary or midnight. Its size depends on the place, date and historical clock rules.
- 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). A summer birth in south-eastern Australia is the same case. The correction can exceed an hour.
- In Singapore and western Malaysia, even a clock time near the middle of a branch can move into the previous branch. Review the calculated time.
- 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 |
|---|---|---|
| Optional city-based calculation (kaucim.ai) | For Singapore, five Malaysian cities and the eight Australian capitals, compares clock time with estimated apparent solar time | Uses approximate city-centre longitude, historical IANA offsets (Australian daylight saving included) and the NOAA equation of time; check precise coordinates near a boundary |
| Manual branch (kaucim.ai) | Keeps your selected date and branch | Use this for other birthplaces, unknown times, or a time already corrected elsewhere |
| 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.
- Treating Australia as one clock. A January birth in Sydney or Melbourne since 1972 is on daylight saving; a January birth in Brisbane or Perth is not, except in the trial years.
- 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?
You can select apparent solar time in the English form for 42 listed US cities, Singapore, five Malaysian cities and the eight Australian capitals. Clock time is the default. The form shows the adjustment, resulting date and branch, then applies that choice to both BaZi and Zi Wei. Saved birth profiles retain the original clock date and branch, so the solar-time choice must be selected again each visit.
How much can the correction be?
There is no universal maximum of ninety minutes. In Singapore, the longitude term is about minus 65 minutes under UTC+8, before the seasonal equation of time. Historical offsets also matter. Immediately before the 1982 change, Singapore and peninsular Malaysia used UTC+7:30. Earlier periods had other offsets. In a Melbourne summer, daylight saving and longitude together come to about minus 80 minutes before the equation of time.
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?
For the listed US cities and the eight Australian capitals, the form reads the historical time-zone offset for the birth date. It asks you to choose an occurrence if the clock time happened twice during a clock change. Elsewhere, 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.
Should a birth in the southern hemisphere reverse the month pillar?
kaucim.ai does not reverse it. BaZi months begin at the twelve major solar terms, and the engine applies them the same way for every birthplace. Some practitioners in Australia read the month six months apart for births south of the equator, so a Yin (寅) month becomes a Shen (申) month, with the month stems recalculated. If another chart differs from ours only in the month pillar, that is the likely reason.
Related concepts
Sources and calculation version
- IANA US time-zone history · historical offsets and daylight saving, including Arizona, Indiana, Hawaii and Alaska
- NOAA solar calculation equations · the approximate equation of time and longitude correction used by the form
- IANA Asian time-zone history · the historical offsets available through the browser; records before 1970 can be incomplete
- IANA Australasian time-zone history · daylight saving by state and year, including the Queensland and Western Australia trials
- NSW Government: daylight saving · the current rule shared by New South Wales, Victoria, South Australia, Tasmania and the ACT
- Chen Yanzhi, 出生南半球的命理研究 (2006) · an Australian argument for reading southern-hemisphere months six months apart
- BaZi calculation rules on kaucim.ai · the rules table on the calculator page
Last checked against the chart engine on 2026-10-03.
For a supported city, enter the original recorded clock time and review the solar-time estimate in the form. For other places, select the appropriate branch manually. If the birth time is uncertain, keep it unknown.
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