Early and late Zi hour
The Zi hour straddles two days: 23:00–24:00 is the “late Zi” (night Zi), 00:00–01:00 the “early Zi”. Whether a late-Zi birth counts as the same or the next day changes the day pillar — and the whole chart. This is the third gate of chart correctness.
1. Where the dispute lies
Traditional calendrics changes the day at midnight (0:00), but folk practice and some schools hold that “the day changes as soon as the Zi hour begins” — after 23:00, count the next day. The two positions directly affect the day pillar and the entire chart of late-Zi births:
| Convention | Rule | Status |
|---|---|---|
| Late Zi to the next day | From 23:00 on, chart by the next day | Mainstream convention; our default |
| Late Zi to the same day | Before 0:00, always chart by the same day | Used by some schools |
2. Our approach: both schools implemented, convention labeled
Both conventions are fully implemented: the mainstream “late Zi to next day” is the default, and “same day” is one click away for comparison. For charts born after 23:00, the convention in use is labeled on the chart — you always know which school's rule produced it.
Engineering-wise, the late Zi hour is a hotspot for boundary bugs (e.g. the Zhongzhou late-Zi Human chart anomaly, fixed in engine v2.5.8); hour boundaries are all covered by our 560-item full-field benchmark.
3. Relation to true solar time
The early/late Zi determination happens after true solar time correction: the clock time is first converted to true solar time, then we judge whether it falls into the late Zi hour and which school's day-change rule applies. The correction order is fixed: time zone → longitude → equation of time → Zi convention (see True solar time correction).