Midnight and Zi-start profiles can select different Day Stems for the same instant.
The Zi branch straddles civil midnight
The twelve traditional hour branches divide a day into two-hour intervals. Under the mapping used by MingWeave, Zi hour runs from 23:00 through 00:59. Civil dates, however, change at 00:00. That leaves the first hour of Zi on one civil date and the second hour on the next.
The Hour Branch can still be Zi across the full interval. The disputed question is when the sexagenary Day Pillar advances. The answer also affects the Hour Stem because the Hour Stem is derived from the Day Stem. A visible Zi branch does not by itself reveal which day convention was used.
Two approved profiles, one explicit difference
The `mw-standard-v1` profile advances the day at civil midnight. A birth at 23:30 uses the current civil date for its Day Pillar. The `mw-zichu-v1` profile advances the sexagenary day at 23:00, so the same instant uses the next day’s Day Pillar. Both keep the same recorded birth input.
True Solar Time variants apply the solar correction before the selected day-boundary rule. This can move an input into or out of the 23:00–00:59 interval. MingWeave warns when the correction crosses an hour or day boundary and shows the corrected clock in its trace.
Read the difference table carefully
For a synthetic 23:30 record away from other boundaries, the Year and Month Pillars should normally remain unchanged between midnight and Zi-start profiles. The Day Pillar changes, and the Hour Stem may change because it is keyed to the Day Stem; the Hour Branch remains Zi. This pattern helps isolate the convention.
If a comparison shows additional changes, inspect solar-time correction, solar-term proximity, daylight-saving resolution, or changed input. A useful comparison names each axis rather than calling the whole chart different. MingWeave’s approved profile IDs encode the time basis and day boundary together.
Handling an uncertain late-night birth
If a record says ‘around 11 p.m.’, clock uncertainty may be as important as the convention. Test times on both sides of 23:00 and midnight, using the same birthplace and time-zone resolution. If the Hour Branch or Day Pillar changes across the plausible range, report both outcomes instead of selecting one for narrative convenience.
If no birth time is known, do not use the calendar date alone to infer a Zi-hour rule. MingWeave returns three pillars and omits time-dependent calculations. The most honest result may be less complete, but it preserves the line between supported calculation and assumption.
- Record the profile ID with any shared chart.
- Separate clock uncertainty from convention uncertainty.
- Never infer an Hour Pillar from a missing time.