True solar time is arguably the most overlooked — yet potentially chart-altering — factor in BaZi. If you were born outside mainland China, or anywhere far from your time zone's standard meridian, understanding this concept is essential for getting an accurate reading.
What Is True Solar Time?
In plain terms, true solar time is what a sundial reads. When the Sun is directly overhead, true solar time is exactly 12:00 noon.
The clock on your wall, however, shows standard time— a political convenience that assigns the same time to everyone within a time zone, regardless of their exact longitude. A single time zone can span 15 degrees of longitude or more, meaning two cities in the same zone can experience "noon" up to an hour apart in solar terms.
Why Clock Time Is Not Solar Time
Two factors cause the discrepancy:
1. Longitude Offset
Every degree of longitude away from your time zone's standard meridian translates to a 4-minute difference in solar time. For instance, China uses a single time zone (UTC+8, standard meridian 120°E). If you were born in Chengdu (roughly 104°E), your true solar time runs about 64 minutes behind Beijing time — over a full hour.
2. The Equation of Time
Because the Earth's orbit is elliptical and its axis is tilted, the Sun does not move at a uniform speed across the sky throughout the year. This causes true solar time to drift between −14 and +16 minutes relative to mean solar time over the course of a year — a factor known as the Equation of Time.
The full correction formula is:
True Solar Time = Clock Time + (Birth Longitude − Zone Standard Longitude) × 4 min + Equation of Time
How This Affects Your BaZi Chart
The Hour Pillar in BaZi is set by the two-hour period (shichen) you were born in. If the true solar time correction shifts your birth across a shichen boundary, the consequences cascade:
- Hour Pillar changes — different Heavenly Stem and Earthly Branch
- Ten Gods shift — a chart typed as "Direct Officer" could become "Indirect Seal"
- Luck Pillars realign — especially the starting age of the first Luck Pillar
- Overall interpretation changes — elemental balance and strength may reverse
A Practical Example
Suppose someone was born in Los Angeles (longitude −118.24°, Pacific Time UTC−8) on June 15, 1990 at 11:50 AM clock time.
By the clock, this falls in the Wu hour (午時, 11:00 AM–1:00 PM). After true solar time correction (longitude offset + Equation of Time), the adjusted time is roughly 11:28 AM — still Wu hour. But if the birth had been just 30 minutes earlier (11:20 AM clock time), the correction could push it into Si hour (巳時, 9:00–11:00 AM), producing an entirely different Hour Pillar.
Who Needs Correction Most?
| Situation | Priority | Reason |
|---|---|---|
| Born near a shichen boundary (e.g. around 11 AM, 1 PM, etc.) | Essential | A few minutes can flip the Hour Pillar |
| Born overseas (U.S., Europe, Australia, etc.) | Strongly recommended | Wide time zones amplify longitude offset |
| Born in western China (Xinjiang, Tibet, Sichuan, etc.) | Strongly recommended | Can be 1–2 hours off from Beijing time |
| Born near the zone's standard meridian | Still advisable | Equation of Time alone can shift 15+ minutes |
How TianJiBu Handles True Solar Time
Our BaZi chart tool includes built-in true solar time correction. Simply enter your birth city or longitude in the advanced settings and the system will automatically:
- Calculate the Equation of Time for your birth date
- Compute the longitude offset from the standard meridian
- Derive your corrected true solar time
- Determine the correct shichen based on the adjusted time
- Generate the chart using the corrected Hour Pillar
If the correction changes your shichen, the results page displays both the pre- and post-correction charts side by side so you can compare.
Frequently Asked Questions
What about Daylight Saving Time (DST)?
If your birth location observed DST at the time you were born, you must first convert the clock time back to standard time before applying the true solar time correction. For example, during U.S. summer DST, subtract one hour from the recorded birth time.
What if I do not know my exact birth time?
If you only know an approximate window (e.g. "morning" or "afternoon"), try generating charts for two or three adjacent shichen and see which one best matches your lived experience. This trial-and-error process — called "time rectification" — is a standard technique in Chinese metaphysics practice.
Does this apply to lunar calendar dates?
True solar time correction adjusts the time, not the date. Whether you enter your birthday in the Gregorian or Chinese lunar calendar, the result is the same as long as the birth time is accurate. However, if the correction pushes the time past midnight (the Zi hour boundary at 23:00), the date itself may need to shift by one day.