Concept
Hour pillar boundaries — and the 23:00 question
Every two hours the chart's hour branch flips. Births near a boundary deserve extra care — and one boundary, 23:00, is a centuries-old debate.
Where the boundaries fall
Boundaries sit at every odd clock hour in solar time: 23:00, 01:00, 03:00 and so on. Note “in solar time” — a birth at 15:02 on the clock may be at 14:47 solar, safely inside the previous branch. This is why boundary questions and true solar time are inseparable: the boundary is fixed, but where your birth falls relative to it moves with the correction.
Births close to a boundary
Within about fifteen minutes of a boundary, small uncertainties — a rounded birth record, an unusual longitude offset — can flip the branch. Our calculator flags this case explicitly instead of presenting a borderline pillar with false confidence. For a flagged birth, reading both candidate hour pillars and seeing which description fits is a legitimate classical practice.
Check the civil-to-solar shift first with the true solar time calculator and visible correction receipt, then use the full chart calculator only if the branch result is relevant.
If another tool gives a different pillar, the BaZi calculator difference checklist helps separate a true-solar-time shift from a different 23:00 day convention.
Questions
Frequently asked
The day divides into twelve two-hour blocks starting with Zi at 23:00–01:00, each named by an earthly branch and paired with a stem derived from the day stem.
For births between 23:00 and midnight, one school starts a new day pillar at 23:00 (Zi begins the day); another keeps the calendar day and changes only at midnight. The two produce different day pillars for the same birth.
The engine implements an explicit school setting rather than hiding one choice; the default follows the mainstream Zi-starts-at-23:00 convention, and the calculation appendix states which rule produced your chart.
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