The corrected clock can change Day or Hour; Year and Month still use the physical instant.
Civil time and apparent solar time answer different questions
Civil time is the clock maintained by a jurisdiction, with daylight-saving and historical changes determined by law. Apparent solar time tracks the observed Sun: apparent noon occurs when the Sun crosses the local meridian. The two clocks need not agree at a given place and date.
A valid True Solar Time calculation begins with a resolved physical instant. It does not ignore historical daylight saving or treat a modern UTC offset as timeless. MingWeave first resolves the local clock through IANA time-zone rules, converts that instant through longitude to Local Mean Solar Time, and then applies the equation of time.
The correction has two parts
Longitude places the physical UTC instant on a local mean-solar clock: Earth rotates roughly one degree in four minutes. The equation of time then accounts for the seasonal difference between mean and apparent solar time caused by Earth’s orbit and axial tilt. The date can move backward or forward through midnight and must not be discarded.
MingWeave keeps the civil time, resolved UTC instant, longitude term, equation-of-time value, and final apparent-solar clock in the trace. The engine uses the project’s versioned formula and explicit precision behavior. That evidence matters because products that say ‘solar time’ may use a different shortcut.
- Historical civil rules resolve the physical UTC instant.
- Longitude converts that instant to local mean solar time.
- Date supplies the equation-of-time correction.
When a correction changes the chart
Most corrections leave the selected date and two-hour branch unchanged. A Day or Hour Pillar changes only when the apparent-solar clock crosses the edge of an hour branch, civil midnight, or 23:00 under the selected day rule. Year and Month always compare the physical UTC birth instant with solar-term instants, so True Solar Time does not move those astronomical boundaries.
For example, a civil time of 08:05 and a negative twenty-minute correction would move to 07:45. That crosses from the Chen branch interval into the Mao branch interval under the standard two-hour mapping, so the Hour Pillar can change. The example is synthetic; a real correction depends on date, longitude, and historical offset.
How to compare the method responsibly
Calculate first with the civil-time standard profile. Then compare with the True Solar Time profile while holding every birth input constant. Check the correction components, boundary warnings, and pillar-by-pillar difference table. Do not change the birthplace or recorded clock time to make a preferred result appear.
Coordinates from a city database represent a locality, not a delivery-room measurement. MingWeave displays a location-precision warning when appropriate. If the corrected time lands very near a boundary, test a plausible coordinate and clock-time range and describe the result as sensitive rather than definitive.