Early / Late Zi Hour 早晚子時 Glossary

The dispute over when the day pillar changes: at civil midnight, or at 23:00 when the Zi hour begins — a question that moves the day pillar for every birth between 23:00 and 01:00.

Zǎo wǎn zǐ shí · 早晚子时 (simplified) · 早晚子時 (traditional)

Source credibility (S/A/B/C/X): A    Rule computability (A/B/C): A

What it is

The twelve double-hours （時辰） each span two clock hours, and the day begins with the Zi hour （子時）, 23:00–01:00. That makes Zi hour structurally awkward: its first half (23:00–24:00, "late Zi" （晚子時）) sits on one civil date and its second half (00:00–01:00, "early Zi" （早子時）) on the next. If the day pillar flips at 23:00, a birth at 23:30 on the 3rd already carries the 4th's day pillar; if it flips at midnight, the 23:30 birth keeps the 3rd's pillar while still receiving the Zi hour. Through the Five Rats escape （五鼠遁）, which derives the hour stem from the day stem, the hour pillar moves with it — so the convention changes two of the four pillars.

Both readings have real lineage support, and software ships both: the "day changes at Zi start" (23:00) convention and the "day changes at midnight" (00:00) convention. This is a disagreement inside the tradition, not a bug in any implementation.

School differences

The project treats this as a registered dispute (entry D-05 in the School Dispute Register): both conventions are implemented, the choice is an explicit parameter in the request, and the register documents that 1,600 dedicated boundary samples in the differential suite verify both paths against the reference implementation. There is no silent default dressed up as orthodoxy.

How this engine computes it

• Both day-boundary strategies are implemented as named parameters (midnight / at23) — entry D-05, "both shipped".
• The engine's default output shows the midnight convention while displaying the 23:00 convention's result alongside, per the boundary-selection table in the kernel specification; the user's choice is recorded with the chart.
• Dedicated boundary samples at 23:30-style timestamps assert that the two strategies produce different (and internally correct) day pillars, so the boundary cannot silently regress.
• The Zi-hour question interacts with True Solar Time: a large solar-time correction can itself push a birth across 23:00 or 00:00, which is why both corrections are shown explicitly.

Related entries

Year Pillar (Lichun boundary) · Chinese New Year · True Solar Time

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