The zi hour and the day boundary: three schools, one choice
The three schools
| School | Day pillar at 23:00–23:59 | Hour stem from |
|---|---|---|
| Split-Zi (分子) | stays on the previous day | the next day's stem |
| Early-Zi (早子) — our choice | advances to the next day | the next day's stem |
| Pure late-Zi (晚子) | stays on the previous day | the previous day's stem |
Several English calculators describe themselves as “late-Zi” while actually implementing split-Zi. Naming the school is the only way to make the comparison possible.
Worked: one evening, three slots
| Slot | Four pillars |
|---|---|
| 亥 slot, 22:30 (same day) | 庚午 辛巳 乙酉 丁亥 |
| 晚子时, 23:30 | 庚午 辛巳 丙戌 戊子 |
| 早子时, next day 00:30 | 庚午 辛巳 丙戌 戊子 |
The late-zi birth (23:30) takes the same day pillar as the following 00:30 birth — that is the early-Zi rule in action. Under split-Zi the day pillar would stay on 20 May, and under pure late-Zi both the pillar and the hour stem would stay.
Why this is where calculators disagree
For births outside 23:00–01:00 the debate is moot. Inside it, the day pillar can move — and because the hour stem is derived from the day stem (Five Rats / 五鼠遁), the hour pillar moves with it. One convention choice therefore changes two of the four pillars.
What our calculator does about it
It declares the school (early-Zi), computes the primary chart under it, and reports the alternative when the school choice changes the answer — rather than silently picking one. See the method page for the full list of declared conventions and the known gaps.
Frequently asked questions
Is a 23:30 birth on Monday or Tuesday?
It depends on the school. Under early-Zi (ours) the day pillar advances at 23:00, so it reads as the next day. Under split-Zi the day pillar stays on Monday but the hour stem comes from Tuesday. Both are in use; neither is a mistake.
Why does the hour stem change too?
The hour stem is derived from the day stem by the Five Rats (五鼠遁) rule. Change the day stem and the hour stem follows, so one boundary decision moves two pillars.
Does the zi hour affect the year or month pillar?
No. Only the day and hour pillars are at stake in the zi-hour debate; the year and month pillars are governed by solar-term instants (see the solar-terms page).
Recompute it yourself
The same engine is published as yilibazi-core on npm (MIT), with source on GitHub — conventions exported as constants, and an ambiguityReport() that returns the alternative chart whenever a convention would change the answer. The stated rules and the known gaps are in the method page.