Many charting tools ask for your birthplace and whether to use "true solar time". Your birth time is on the certificate — so why convert it?
Double-hours follow the sun
Ancient double-hours were defined by the sun's position. When the sun is due south and shadows are shortest, it is local noon — the middle of the Wu hour. The BaZi hour pillar rests on this sun-based time, so a traditional reading needs the local solar time of birth.
Standard, mean and true solar time
Standard time
China uses Beijing time everywhere, based on mean solar time at 120°E.
Local mean solar time
The earth turns 360° in 24 hours, so each degree of longitude is about 4 minutes. West of 120°E, local solar time is behind Beijing time; east of it, ahead. Urumqi (about 87.6°E) is roughly 32° west — over two hours behind. A clock hour there may still be the previous double-hour.
True solar time
Even at one longitude, solar noon shifts during the year because the orbit is elliptical and the axis tilted — the equation of time, varying within about ±15 minutes. Longitude plus equation of time gives true solar time.
How much does it matter?
For most people the double-hour does not change. Watch for:
- Times near a boundary, like 06:55 or 10:58.
- Birthplaces far from the reference meridian, e.g. Xinjiang or Tibet, where the gap can exceed an hour.
- Births between 23:00 and 01:00, which may change the day pillar.
- Births near a solar-term change, which can even affect the month pillar.
Daylight saving time
China used DST from 1986 to 1991; summer clocks ran one hour ahead, so subtract an hour first. If you were born abroad, check local DST too.
Opinions differ
Practitioners don't fully agree. Most modern enthusiasts favor true solar time as closer to the original meaning; others prefer the time society actually used. Knowing both and comparing is a sensible approach.
Summary
True solar time accounts for longitude and the seasonal equation of time. It rarely changes a chart, but for people born near a boundary or far from the reference meridian it can change the hour pillar.
This article is for cultural reference only and is not advice for any decision.