Earthly Branches and Double Hours (地支與時辰)
Map the twelve two-hour periods from Zi through Hai, document the time standard used, and handle midnight and true-solar-time conventions transparently.
Overview
The standard civil-time table assigns Zi to 23:00–00:59, Chou to 01:00–02:59, and continues in two-hour blocks through Hai at 21:00–22:59. A reliable calculator must state whether it uses civil time, local mean solar time, true solar time, or another historical convention, because location and boundary choices can change the hour pillar.
At a glance
- Zi and Chou
- 23:00–00:59; 01:00–02:59
- Yin through Si
- 03:00–10:59 in four two-hour blocks
- Wu through You
- 11:00–18:59 in four two-hour blocks
- Xu and Hai
- 19:00–20:59; 21:00–22:59
- Required setting
- Civil, mean-solar, true-solar, or documented alternative
- Boundary risk
- Midnight and exact block edges need explicit handling
Start from a complete twelve-block table
The standard mapping divides a 24-hour day into twelve named intervals, each two civil hours wide.
Store start and end instants rather than relying on a display label, so exact boundaries can be tested and reproduced.
- Use all twelve intervals.
- Define inclusive and exclusive boundaries.
Declare the time standard before conversion
Civil clock time reflects time zones and legal offsets; solar-time methods additionally use longitude and, for true solar time, the equation of time.
No single setting should be silently presented as universal tradition. The selected method and any correction must be visible to the user.
- Preserve the original civil timestamp.
- Show every applied correction.
Handle the Zi-hour day boundary separately
Schools differ on whether the day stem changes at 23:00, midnight, or another defined boundary while Zi hour spans midnight.
An engine must choose one documented rule, expose it, and avoid combining a day stem from one convention with an hour stem from another.
- Keep day rollover and hour branch distinct.
- Test both sides of midnight.
An hour branch does not prove an event or trait
The hour branch can provide chart position, hidden stems, and relation data, but it does not independently reveal children, private behavior, career outcome, or lifespan.
Uncertain or estimated birth times should remain marked as uncertain and should not drive consequential advice.
- Do not hide estimated-time uncertainty.
- Avoid private or high-stakes inference.
Sources and editorial basis
- Hong Kong Observatory: Heavenly Stems and Earthly Branches香港天文台「天干地支」：一日十二時辰與地支相配，每時辰約為現代兩小時An official modern reference for the canonical stem and branch orders, the sexagenary cycle, and calendrical use.
- 《御定星曆考原》卷一《星曆考原》時辰與日界諸論：時支換算需先固定所採時間與換日規則A historical source for stem and branch Yin-Yang, Five Phase, direction, seasonal, and calendrical correspondences.
- OpenFate Editorial Methodology十二時辰頁保存原始時刻、時區及補正前後值，明示子時流派差與出生時刻不確定性OpenFate separates deterministic chart calculation, traditional interpretation, and modern editorial explanation.