What True Solar Time Is
True solar time follows the sun at your birthplace, while clock time only follows your time zone. BaZi and Ziwei Doushu both take the birth hour from true solar time. That hour is one of twelve two-hour periods (shichen), not a clock hour.
Enter your birth date, time and city in the converter below to see your true solar time and period. Pick your birth city in the free BaZi calculator and it applies the correction for you.
True Solar Time Calculator
Enter your birth date, time, and city to convert clock time to true solar time and see whether your shichen (two-hour period) changes.
Calculate your chart
Ziwei Doushu maps your stars across twelve palaces; BaZi reads your Day Master and elemental balance. Both calculators are free.
How It Works: Longitude Plus the Equation of Time
True solar time is clock time plus a longitude correction plus the equation of time. At 12 noon on the clock, the sun is not always due south where you are. Within one time zone, the sun reaches places in the east earlier and places in the west later.
- Undo daylight savingIf daylight saving was in force on your birth date, turn the clock back one hour first.
- Longitude correctionEach degree between your birthplace and the time zone's central meridian is worth 4 minutes. For Taiwan and Hong Kong, that meridian is 120°E.
- Equation of timeThe earth moves around the sun faster at some times of year than others. This part changes with the date, from about −15 to +16 minutes.
- Find the periodLook up the two-hour period for the corrected time. After 11 p.m., the birth counts as the next day.
A time corrected for longitude only, without the equation of time, is called local mean time.
How Many Minutes to Subtract for a Hong Kong Birth
For a Hong Kong birth, subtract about 7 to 38 minutes, depending on the date. Hong Kong lies at about 114.2°E, so longitude alone takes off about 23 minutes. The equation of time then takes off the most in mid-February and the least in late October.
| Birth date | Equation of time | Hong Kong total |
|---|---|---|
| Jan 15 | −10 min | −33 min |
| Feb 15 | −15 min | −38 min |
| Mar 15 | −10 min | −33 min |
| Apr 15 | −1 min | −24 min |
| May 15 | +3 min | −20 min |
| Jun 15 | −1 min | −24 min |
| Jul 15 | −6 min | −29 min |
| Aug 15 | −4 min | −27 min |
| Sep 15 | +5 min | −18 min |
| Oct 15 | +15 min | −8 min |
| Nov 15 | +15 min | −8 min |
| Dec 15 | +4 min | −19 min |
Say you were born in Hong Kong at 9:20 a.m. on 12 February 1990. By the clock, that is the Si period. After a −38-minute correction it becomes 8:42 a.m., which falls in the Chen period.
Hong Kong used summer time in 1941, from 1946 to 1976, and in 1979. During the Japanese occupation from 1942 to 1945, clocks ran one hour ahead all year. The converter undoes all of these for you.
Does a Taiwan Birth Need Correcting?
Yes, but on Taiwan's main island the correction is usually small. Longitude there adds only about +1 to +7 minutes. With the equation of time, the total runs from about −14 to +23 minutes over the year.
Your period is only likely to change if you were born within 25 minutes of a boundary. A birth at 8:50 a.m., for example, may pass 9:00 after correction and move from the Chen period to the Si period.
Taiwan observed daylight saving in the summers of 1946–1961, 1974–1975 and 1979. Enter the time on your birth certificate as it is, and the converter undoes it for you.
Longitude Corrections for Major Cities
The longitude correction depends only on the birth city, not the date. Taiwan's main island needs almost none. Hong Kong and Macau lose over twenty minutes, while Singapore and Malaysia lose more than an hour.
| City | Time zone | Longitude | Correction |
|---|---|---|---|
| Taipei | UTC+8 | 121.56°E | +6 min |
| Keelung | UTC+8 | 121.74°E | +7 min |
| Taichung | UTC+8 | 120.68°E | +3 min |
| Tainan | UTC+8 | 120.21°E | +1 min |
| Kaohsiung | UTC+8 | 120.31°E | +1 min |
| Hong Kong | UTC+8 | 114.17°E | −23 min |
| Macau | UTC+8 | 113.54°E | −26 min |
| Singapore | UTC+8 | 103.85°E | −65 min |
| Kuala Lumpur | UTC+8 | 101.69°E | −73 min |
| Penang | UTC+8 | 100.33°E | −79 min |
| Shanghai | UTC+8 | 121.47°E | +6 min |
| Beijing | UTC+8 | 116.39°E | −14 min |
| Shenzhen | UTC+8 | 114.06°E | −24 min |
| Guangzhou | UTC+8 | 113.26°E | −27 min |
| Chengdu | UTC+8 | 104.07°E | −64 min |
| Xi'an | UTC+8 | 108.94°E | −44 min |
| Urumqi | UTC+8 | 87.62°E | −130 min |
| New York | UTC−5 | 74.01°W | +4 min |
| Los Angeles | UTC−8 | 118.24°W | +7 min |
| London | UTC+0 | 0.12°W | 0 min |
| Sydney | UTC+10 | 151.21°E | +5 min |
| Vancouver | UTC−8 | 123.12°W | −12 min |
Add the equation of time for your birth date to these numbers. Overseas cities are listed in standard time, and the converter undoes daylight saving for you.
Two-Hour Periods and Births After 11 p.m.
Once true solar time passes 11 p.m., the birth falls in the Zi period and the date moves to the next day.
Say you were born in Taipei at 10:50 p.m. on 3 November 1990. By the clock, that is the Hai period. After a +22-minute correction it is 11:12 p.m., the Zi period, so the date counts as 4 November.
Some practitioners still count 11 p.m. to midnight as the same day, which gives a different day pillar. Each of the twelve periods lasts two hours and matches one of the twelve Earthly Branches.
| Period | True solar time |
|---|---|
| Zi | 23:00–00:59 |
| Chou | 01:00–02:59 |
| Yin | 03:00–04:59 |
| Mao | 05:00–06:59 |
| Chen | 07:00–08:59 |
| Si | 09:00–10:59 |
| Wu | 11:00–12:59 |
| Wei | 13:00–14:59 |
| Shen | 15:00–16:59 |
| You | 17:00–18:59 |
| Xu | 19:00–20:59 |
| Hai | 21:00–22:59 |
What True Solar Time Changes in BaZi and Ziwei Doushu
When the two-hour period changes, the BaZi hour pillar changes. Cross 11 p.m. and the day pillar changes too. A BaZi chart is four pillars for year, month, day and hour, so changing one gives a different chart.
In Ziwei Doushu, the Life Palace moves to another cell, and the other palaces shift with it. The stars placed by birth hour move as well: Wenchang, Wenqu, Huoxing, Lingxing, Dikong and Dijie.
When the Life Palace moves, the fourteen major stars, Ziwei among them, may move too. Crossing 11 p.m. also moves the lunar date by a day, which can shift them as well. To see what each system looks at, read Ziwei Doushu vs BaZi.
