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Orphic Meridian Association

Origins of Symbols · Calendars and Bazi

FDL-023

When Does the Bazi Day Change During the Zi Hour?

Zi double hour crossing midnight with two day boundaries beside a water clock
Zi hour spans about 23:00 to 01:00, allowing a boundary at its start or midpoint. Generated illustration.
Author
Editorial Board, Orphic Meridian Association
Date
2026-04-08
Reading time
3 min

Civil dates change at 00:00, while the Zi double hour commonly spans approximately 23:00 to 01:00. Because one traditional time unit crosses midnight, Four Pillars methods disagree over when the day pillar changes.

Two common rules

The early-Zi rule begins the new day at 23:00 and assigns the whole double hour to it. The midnight rule changes at 00:00, treating 23:00–23:59 as late Zi of the previous day and 00:00–00:59 as early Zi of the next.

Some methods retain the previous day stem during late Zi while deriving the hour stem under another convention. Results near 23:00 therefore require more than a calendar lookup.

Historical uses did not require one boundary

Observation, water clocks, official documents, calendar notation, and daily life could organize time differently. Later technical traditions inherited terms while explaining their boundaries in different ways.

Agreement among websites may simply show a shared library or default. Test a 23:30 case, inspect the setting, and verify both day and hour stems. Also check whether civil or corrected solar time is displayed, because a correction may cross the boundary.

State the convention

Write, for example: “This chart changes the day at midnight; 23:00–23:59 remains on the previous day.” If the source says only “Zi hour,” preserve both possibilities.

Time correction and boundary rules interact

A recorded 23:10 may move earlier or later when a method converts civil time to local or apparent solar time. The corrected value could leave or enter the disputed interval. Checking only the day-boundary option while ignoring the time standard produces a false comparison.

Keep a log with the original timestamp, every correction, the final working time, and the boundary rule. When the source is imprecise, identify which conclusions remain stable under both possible charts.

See why birth time standards can change the hour pillar and how the sixty-day cycle works.

References