Zhen Tai Yang Shi (True Solar Time): The Time Standard for Accurate Charts

Definition

True Solar Time is time based on the sun's actual position in the local sky — when the sun is at its highest (upper meridian), it is local noon. It usually differs from the clock time we use daily (the uniform standard-zone time). BaZi charts are cast on true solar time so the hour pillar is set correctly.

Why Correction Is Needed

Clock time is fixed uniformly by time zone (e.g. all of China uses UTC+8 Beijing time), but every location's longitude differs, so the sun reaches local noon at a different real moment. The difference comes from two parts: - **Longitude offset:** the gap between your longitude and the zone's standard meridian — about 4 minutes per degree of longitude; - **Equation of time:** a seasonal variation from Earth's orbital eccentricity and axial tilt, swinging roughly −14 to +16 minutes across the year. Together, clock time and true solar time can differ by tens of minutes.

Why It Matters

- If a birth moment is near an hour-branch boundary, correcting (or not) can change the **hour pillar**, and thus the whole chart and the [Life Palace](/bazi/glossary/ming-gong). - The hour pillar governs later life, children, and many judgments — a wrong hour skews everything.

Common Misconceptions

**1: Just use the clock time on the birth certificate.** Rigorous casting corrects to true solar time using birthplace longitude plus the equation of time. **2: No need to distinguish early/late Zi hour.** The Zi hour straddles midnight; whether the day rolls over must be handled carefully by school and true solar time.

Significance in Chart Analysis

True solar time is the prerequisite for an accurate chart. This site supports birthplace and time correction — use the [free BaZi calculator](/bazi/chart) and learn about the hour and Life Palace in the [BaZi learning center](/bazi/learn).