Early and Late Zi Hour (早晚子時) in Bazi
Understand early and late Zi Hour, the 23:00 versus midnight day-boundary doctrines, and how OpenFate calculates the day and hour pillars.
Overview
Early and Late Zi Hour (早晚子時) divides the two-hour Zi period, 23:00–00:59, around midnight. The distinction matters because Four-Pillars schools do not all change the day pillar at the same instant. OpenFate supports an explicit 23:00 rollover mode and a midnight rollover mode; in the latter, 23:00–23:59 keeps the civil-date day pillar while the Zi-hour stem follows the next day. Time zone and daylight-saving history must be resolved before either doctrine is applied.
At a glance
- Zi interval
- 23:00–00:59 local normalized time
- Late Zi
- 23:00–23:59
- Early Zi
- 00:00–00:59
- Supported doctrines
- 23:00 rollover or 00:00 rollover
- Calculation order
- Time zone and DST before day-boundary rule
- Boundary
- Convention changes pillars, not lived events
Early and late Zi name two halves of one branch hour
The Zi branch covers two civil-clock hours that cross midnight. Late Zi denotes the hour before midnight; early Zi denotes the hour after it.
These labels solve a calendrical question. They are not two additional branches, personality types or favorable and unfavorable time grades.
- Record the exact local time.
- Keep the branch hour separate from the day-rollover rule.
The 23:00 and midnight doctrines produce different day pillars
Under a 23:00 rollover, the new sexagenary day begins when Zi hour begins. A birth at 23:30 therefore uses the following day for both day and hour stem derivation.
Under a midnight rollover, 23:30 retains the current civil-date day pillar, while its Zi-hour stem follows the next day; 00:30 is early Zi and uses the new civil day consistently.
- Declare the selected doctrine in the result.
- Never silently combine outputs from two doctrines.
OpenFate applies civil-time normalization before the doctrine
The engine first resolves location, historical UTC offset and daylight-saving status. It then evaluates the local clock against the selected day-boundary mode.
Regression tests pin 22:59 as Hai hour, both Zi halves, and midnight rollover behavior so a display or content change cannot alter pillar calculation.
- Normalize the birth instant before classification.
- Version the convention with the saved calculation.
A boundary convention cannot predict an event by itself
Changing the doctrine may alter the day pillar, Day Master and downstream relations for a boundary birth. The result should be compared transparently rather than choosing the more flattering chart.
Neither mode proves character, health, marriage, wealth or fate. If the recorded time is uncertain, preserve that uncertainty and compare the plausible charts.
- Show sensitivity when the birth time is near the boundary.
- Do not select a doctrine to force a desired reading.
Sources and editorial basis
- 《御定星曆考原》卷一《御定星曆考原》卷一干支時刻與曆日配屬：子時跨越民用午夜，日柱換界必須聲明所採曆法約定A historical source for stem and branch Yin-Yang, Five Phase, direction, seasonal, and calendrical correspondences.
- IANA Time Zone DatabaseIANA Time Zone Database：先依出生地與歷史日期還原當地民用時刻及 UTC 偏移，再判斷 23:00 或 00:00 換日The maintained reference for historical local-time rules, UTC offsets, and daylight-saving transitions by location.
- OpenFate Editorial Methodology產品 day-boundary doctrine 回歸測試固定 ZI_HOUR_23 與 MIDNIGHT_00 兩模式、22:59 亥時及早晚子時；模式只改計算，不自動生成事件結論OpenFate separates deterministic chart calculation, traditional interpretation, and modern editorial explanation.