True Solar Time Bazi Calculator

The short answer: how is true solar time calculated?

This calculator uses the following structure: true solar time = recorded local civil time + longitude correction + equation of time.

• Longitude correction (minutes) = 4 × (birthplace longitude − reference meridian derived from the UTC offset at that instant). Earth rotates roughly 360° in 24 hours, so one degree corresponds to about four minutes.
• Reference meridian (degrees) = UTC offset at the birth time in minutes ÷ 4. This is a calculation reference derived from the offset at that instant, not a claim that the region changed its legal central meridian. The calculator looks up the IANA timezone history for the birth date, including applicable daylight-saving rules.
• The equation of time represents the seasonal difference between apparent and mean solar time. The current tool uses a lightweight approximation: B = 2π × (day of year − 81) ÷ 364; E = 9.87sin(2B) − 7.53cos(B) − 1.5sin(B).
• The combined correction is rounded to the nearest whole minute before the Four Pillars are recalculated. If the result lies within one minute of a day or hour boundary, compare the adjacent result and the original birth record rather than relying on the rounded time alone.

Why can true solar time matter in a Bazi chart?

Timezone and DST describe civil clock rules; longitude and the equation of time describe solar time. The historical UTC offset already includes DST, so do not subtract it again. Read the time-calibration guide for birth-record checks and day-boundary conventions.

An Hour Pillar normally covers a two-hour branch period. Some Bazi conventions also change the Day Pillar at 23:00, the start of the Zi hour. If the recorded time is close to 23:00, midnight, or another two-hour boundary, longitude and equation-of-time corrections may move it across that boundary and change the Day or Hour Pillar. Year and Month Pillars are compared against the solar-term instant on one absolute timeline; rewriting that same instant as a true-solar clock reading does not move it backward into another solar-term month. Away from a boundary, the corrected time can differ while the Four Pillars remain unchanged.

That is why this page shows both results. You can verify how many minutes were applied and whether the chart itself changes, instead of receiving a single opaque answer.

How precise is this result?

It is designed for minute-level comparison before a Bazi calculation. It is not an astronomical observation or legal time certificate:

1. Coordinates represent a city, district, or county reference point rather than a specific building.
2. The IANA Time Zone Database is one major reference for modern civil-time and daylight-saving data. Its pre-1970 coverage is limited, so earlier or disputed records still require local official sources.
3. The equation of time uses an approximation and the final correction is rounded to a whole minute, not calculated from a high-precision solar ephemeris.
4. Uncertainty in the recorded birth time may be larger than the formula error. Near a boundary, retain both results and check the most reliable birth record available.

Frequently asked questions

Is there one fixed correction for every birth in China?

No. China uses UTC+8, corresponding to the 120°E reference meridian, but cities have different longitudes and the equation of time changes with the date. A fixed “minus 14 minutes” rule is therefore not valid everywhere or every day.

Does true solar time always change the Hour Pillar?

No. A pillar changes only when the corrected time crosses a relevant branch or day boundary. The result panel identifies that condition explicitly.

How are overseas daylight-saving rules handled?

Choose a matched overseas city. The tool asks the IANA timezone for the UTC offset on the birth date rather than using today's offset, so historical daylight-saving rules for places such as New York, London, and Sydney are included. If a local time was skipped in spring or repeated during a fall rollback, the tool stops and asks you to verify the record instead of silently choosing an instant.

Method and primary sources

• PNNL Technical Report PNNL-30028: Appendix A directly documents the 364-day equation-of-time approximation used on this page.
• NOAA Solar Calculator equations: documents the relationship among total solar-time offset, longitude, UTC offset, and equation of time; NOAA uses a different equation-of-time approximation.
• IANA Time Zone Database: one major reference for modern civil-time and daylight-saving data; earlier or disputed records still require local official sources.

The astronomical time-conversion relationship comes from modern solar and civil-time sources; applying true solar time to Bazi Day and Hour Pillars is a charting convention. Bazi schools differ on whether to apply true solar time and whether a day changes at 23:00 or midnight. This tool exposes both boundary options and does not present a lineage choice as a single scientific fact. See the full Bazi calculation method and the reproducible validation cases.