True Solar Time in BaZi: Clocks Serve a Whole Region, a Chart Follows the Sun
The ancients read the hour from a sundial: when the sun stood due south, it was the middle of the Wu hour. A modern clock follows one standard meridian for an entire country, sometimes several, and that does not always match where the sun is over your birthplace. True solar time puts those two gaps back.
The formula
True solar time = standard time + longitude offset + equation of time
- Longitude offset: the birthplace's longitude minus the time zone's standard meridian (120°E for Taiwan, Hong Kong and mainland China), four minutes per degree: add it east of the meridian, subtract it west.
- Equation of time: Earth's orbit is an ellipse and its axis is tilted, so the moment the sun crosses due south drifts earlier and later through the year. It is furthest behind in mid-February (about −14.6 minutes) and furthest ahead in late October and early November (about +16.5 minutes).
- If you were born under daylight saving time, take that hour off first; see daylight saving time.
Longitude offset for cities in Taiwan
| City | Longitude | Offset | City | Longitude | Offset |
|---|---|---|---|---|---|
| Keelung | 121.74°E | +7 min | Taipei | 121.56°E | +6.2 min |
| New Taipei | 121.46°E | +5.8 min | Taoyuan | 121.3°E | +5.2 min |
| Hsinchu | 120.97°E | +3.9 min | Miaoli | 120.82°E | +3.3 min |
| Taichung | 120.68°E | +2.7 min | Changhua | 120.54°E | +2.2 min |
| Nantou | 120.69°E | +2.8 min | Yunlin | 120.54°E | +2.2 min |
| Chiayi | 120.45°E | +1.8 min | Tainan | 120.21°E | +0.8 min |
| Kaohsiung | 120.3°E | +1.2 min | Pingtung | 120.49°E | +2 min |
| Yilan | 121.75°E | +7 min | Hualien | 121.6°E | +6.4 min |
| Taitung | 121.14°E | +4.6 min | Penghu | 119.57°E | −1.7 min |
| Kinmen | 118.32°E | −6.7 min | Matsu | 119.95°E | −0.2 min |
On Taiwan's main island the offset runs from about +1 to +7 minutes, a little more in the east and north, while Kinmen is −7. For a Taiwan birth, longitude barely matters; the equation of time is what counts.
The equation of time by month (minutes)
| Month | 1st | 15th |
|---|---|---|
| January | −4 | −9 |
| February | −14 | −15 |
| March | −13 | −10 |
| April | −4 | 0 |
| May | +3 | +4 |
| June | +2 | 0 |
| July | −4 | −6 |
| August | −6 | −4 |
| September | +1 | +6 |
| October | +11 | +15 |
| November | +16 | +15 |
| December | +10 | +4 |
How we counted: our engine drew random birth times (to the minute) within each period, 12,000 per group, and compared the four pillars cast straight from the clock hour with the exact, corrected pillars. The modern Taiwan group was born 1980–2010; the daylight-saving group, 1946–1979.
The equation of time: no daylight saving, and the hour still changes
Born November 3, 2000, 0:40 a.m., Taipei
- Longitude: Taipei lies east of 120°E, so add 6 minutes.
- Equation of time: early November is when the sun runs furthest ahead of the clock all year; the equation of time is about +16 minutes.
- Result: true solar time is 01:02, just past 1 a.m. The birth moves from the Early Zi hour into the Chou hour, and the hour pillar changes from Bing-Zi (丙子) to Ding-Chou (丁丑). It sits only 2 minutes from the boundary, so how accurately the time was recorded matters more than the correction itself.
Born in mainland China or Southeast Asia: much larger gaps
| Birthplace | Region | Longitude offset |
|---|---|---|
| Hong Kong | Hong Kong & Macau | −23 min |
| Macau | Hong Kong & Macau | −26 min |
| Xiamen | Mainland China | −8 min |
| Shanghai | Mainland China | +6 min |
| Beijing | Mainland China | −14 min |
| Guangzhou | Mainland China | −27 min |
| Chengdu | Mainland China | −64 min |
| Urumqi | Mainland China | −129 min |
| Singapore | Southeast Asia | −65 min |
| Kuala Lumpur | Southeast Asia | −73 min |
| Tokyo | Northeast Asia | +19 min |
| Seoul | Northeast Asia | −32 min |
All of mainland China runs on Beijing time, so western cities such as Chengdu, Kunming and Urumqi are one to more than two hours behind the clock. Singapore and Kuala Lumpur switched to UTC+8 in 1982, putting the clock more than an hour ahead of the sun. If you were born in one of these places and do not correct, the hour pillar is almost certainly wrong.
The same arithmetic works anywhere. Wide time zones are the ones to watch: western Spain and France keep Central European Time, a long way east of their own longitude, and cities on the western edge of a US time zone run well behind their clocks. The tool on this page includes a handful of cities in North America, London, Australia and New Zealand; for anywhere else, pick the one closest in longitude.
Should you correct? Both sides have a case
| Correct for true solar time | Use clock time | |
|---|---|---|
| The argument | The hours were defined by the sun in the first place, and the classical Zi Ping method uses local solar time | Birth times are often ten or more minutes out, so precise correction is lost in a rough record |
| Best for | Hospital records to the minute; birthplaces far from the standard meridian | Times remembered by family, known only roughly; places close to their meridian, such as Taiwan |
What we do: the home page chart form does not correct; the in-depth reading corrects automatically when you give an exact time and birthplace; and the tool on this page shows the before and after side by side, so you can compare for yourself.
Correct your birth time
Enter the Gregorian date, time and place from the birth certificate. The tool works through daylight saving time, the longitude offset, the equation of time, the Zi-hour day change and the solar term boundary in one go, then compares the result pillar by pillar with a chart cast straight from the clock hour. Everything is calculated in your browser; nothing is uploaded.
Enter the clock time exactly as the birth certificate shows it; the tool takes off daylight saving time itself, so do not subtract it first. The day changes at 11 p.m. If your birthplace is not listed, pick the city closest in longitude (one degree is only four minutes).
A straight word: correction only helps if the recorded time is accurate
Hospital records usually give the minute, but family memory is more often "a bit after nine in the morning." If the time could be ten or twenty minutes out, fine-tuning it is pointless. If your time is an estimate, rather than worrying about whether to correct it, cast both possible hours and see which chart sounds more like you. Everything on this site is for reference and entertainment.
Work out your true solar time
Enter the time and place from your birth certificate, and the tool lists the longitude offset, the equation of time and the corrected hour.
True solar time FAQ
How is true solar time calculated?
Standard time plus the longitude offset plus the equation of time. The longitude offset is the birthplace's longitude minus the standard meridian, at four minutes per degree; the equation of time varies with the date between about −14.6 and +16.5 minutes.
How far is Taipei from true solar time?
Taipei lies at 121.56°E, a longitude offset of about +6 minutes. Add the equation of time for the day, and over a year the total runs from about −8 to +23 minutes.
Does BaZi have to use true solar time?
It is a choice between schools. Those who follow the classical method use true solar time; other teachers use clock time because birth records carry their own error. Correction matters most for people with a time recorded to the minute and a birthplace far from the standard meridian.
Should people born in Singapore or Malaysia correct for it?
We would say yes. Both have used UTC+8 since 1982, but they lie at only about 101° to 104°E, so the clock runs more than an hour ahead of the sun, and without correction the hour pillar is often wrong.