Why the clock is not the Sun
The twelve Bazi double-hours were defined long before time zones. Noon, the middle of the 午 hour, was the moment the Sun stood highest in the local sky. Modern clocks instead keep one time across a whole zone, often for political reasons. All of China, which spans more than sixty degrees of longitude, keeps Beijing time; Spain keeps Central European time although it lies mostly west of Greenwich. Whenever a birthplace is far from its zone’s reference meridian, clock time and solar time differ.
A second, smaller difference comes from the Earth’s orbit. Because the orbit is an ellipse and the Earth’s axis is tilted, the Sun does not reach its highest point at the same clock time every day: it is up to about 16 minutes early in early November and about 14 minutes late in mid-February. This is the equation of time.
True solar time (真太阳时) is clock time corrected for both effects. The classical texts did not have the modern vocabulary, but they did insist that place matters. The Di Tian Sui commentary on the hour of birth says that among people born at the same date and hour, one must also consider the differences of “mountains and rivers”:
凡同年月日時，而百人各一應者，固當究其時之先後，又當論其山川之異“When a hundred people share the same year, month, day and hour and each turns out differently, one should certainly investigate the earlier or later part of the hour, and also consider the differences of mountains and rivers.”— Di Tian Sui (Drops from Heaven’s Marrow) 《滴天髓》, 生時論. Original text: Wikisource · also on ctext.org (different edition). Translation: bazimethod.
The conversion, step by step
- Clock time to universal time. Subtract the time-zone offset that applied at that place on that date — including daylight saving if it was in force. This is where historical records matter (see below).
- Longitude correction. The Sun moves 15° of longitude per hour, so 4 minutes per degree. Local mean time = universal time + longitude × 4 minutes (east positive).
- Equation of time. Add the equation of time for that date (between about −14 and +16 minutes) to get true solar time.
Our calculator performs these three steps and prints each one, so you can check them. We use the IANA time-zone database, built into every modern browser, for step 1, and the NOAA formula for the equation of time in step 3.
Worked examples
Urumqi, 12:00 Beijing time on 1 June
Urumqi lies at about 87.6° E but officially keeps Beijing time (UTC+8, reference meridian 120° E). The longitude correction is (87.6 − 120) × 4 ≈ −130 minutes. The equation of time on 1 June is about +2 minutes. So 12:00 on the clock is 09:52 true solar time — the 巳 hour, not the 午 hour that the clock suggests. This case is one of our automated tests. (A practical warning: the time-zone database lists Urumqi under “Xinjiang time”, UTC+6, which is widely used locally. If a birth record uses Beijing time, as official records normally do, set the time zone to Asia/Shanghai in the calculator’s options; the calculator reminds you of this.)
Shanghai, 14:30 on 15 May 1990
China used daylight saving time from 1986 to 1991. On 15 May 1990 clocks in Shanghai were at UTC+9, so 14:30 on the clock was 05:30 UTC. Shanghai is at about 121.5° E, giving local mean time of about 13:36, and the equation of time adds about 4 minutes: true solar time ≈ 13:39, in the 未 hour. Forget the daylight-saving step and you get 14:39 — still 未 in this case. But for a birth at 15:30 on the clock, the correct answer is 14:39 (未) while ignoring daylight saving gives 15:39 (申): a different hour pillar.
Madrid, 08:00 in January
Madrid is at about 3.7° W and keeps UTC+1 in winter. Longitude correction relative to the zone: (−3.7 − 15) × 4 ≈ −75 minutes. Equation of time on 15 January ≈ −9 minutes. 08:00 on the clock is about 06:36 solar time — the 卯 hour, not the 辰 hour.
Historical time zones and daylight saving
The time-zone step is easy to get wrong because rules change. Examples that affect many charts:
- China 1986–1991: daylight saving each summer.
- Hong Kong, Taiwan and many other places used summer time in various years of the mid-twentieth century.
- Before standard time zones (before roughly 1900 in most places), clocks kept local mean time; the IANA database represents this with the local mean time of the zone’s main city (Shanghai for Asia/Shanghai, Berlin for Europe/Berlin), not of your birthplace. When we detect such an entry, our calculator treats the time you enter as local mean time at the birthplace itself.
- Wars and occupations changed clocks abruptly in many countries.
We rely on the IANA database, which is maintained by volunteers who document their sources, and is the standard used by operating systems. It is very good, but historical local practice was not always uniform; if your birth record states which time was used, trust it.
Ambiguous and impossible times
When clocks go back, one hour occurs twice; when they go forward, one hour never exists. Our calculator detects both: for repeated times it lets you choose the first or second occurrence, and for non-existent times it warns you and treats the time as standard time.
Which pillars does solar time affect?
True solar time affects the hour pillar most often and the day pillar for births near midnight. It does not affect the year and month pillars in our method: those change at the exact astronomical moment of a solar term, which is the same instant everywhere on Earth. We therefore compute year and month from the absolute instant of birth, and day and hour from local solar time. Some calculators instead apply solar-term boundaries in local time; near a boundary that can give a different month pillar.
Should you use it at all?
Most classical practitioners and serious modern ones use local solar time. Many printed almanacs and casual apps use clock time, simply because it is easier. Both conventions are in use; ours defaults to true solar time and lets you switch it off. If the two settings give different hour pillars for you, your chart sits near a boundary, and the accuracy of the recorded birth time matters more than the convention.
Calculate your chart to see your own correction.
Keep learning
More in Bazi basics, plus core guides — or go back to the Bazi calculator home page.
Classical quotations are copied from the public-domain texts on Wikisource; links let you check every line. English translations are our own. Traditional Bazi statements are reported as tradition, not as fact.