Chart methods and reference
Does the BaZi day change at 23:00 or midnight? Zi-hour boundaries
“Zi hour begins at 23:00” and “the day pillar changes at 23:00” are separate rules. Zi hour crosses midnight. When comparing calculators, inspect both the day and the hour pillar.
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Calculate my BaZi chartA two-hour period and a day boundary are separate choices
Modern calculators generally assign 23:00 through 00:59 to Zi hour. The first hour is often called late Zi and the hour after midnight early Zi. Some systems assign the next day pillar from 23:00; others change it at 00:00. There are also separate conventions for deriving early and late Zi hour stems. A label such as “split Zi hour” is therefore insufficient: record which date supplies the day stem and which supplies the hour stem.
The precise rule implemented here
The Go engine first resolves the corrected true solar calendar date. Its day pillar changes at midnight. Both 23:00 and 00:00 are Zi hour, but each hour stem is derived from its own calendar day stem using the Five Rat rule. The branch can therefore stay Zi while its stem changes at midnight. The chart currently has no switch for a 23:00 day boundary; this article explains the alternative without presenting it as an available feature.
Paired inputs across 23:00 and then midnight
The table treats Gregorian Beijing clock readings as standard time, omitting solar correction to isolate the boundary logic. On 2024-06-15, 22:59 and 23:00 share the Geng Xu day, while the hour changes from Ding Hai to Bing Zi. At 2024-06-16 00:00, the result is Xin Hai day and Wu Zi hour. A calculator that advances both date and hour-stem reference at 23:00 would show the latter pair earlier. Compare conventions before declaring an error.
| Time (UTC+08:00) | Year pillar | Month pillar | Day pillar | Hour pillar |
|---|---|---|---|---|
| 2024-06-15 22:59 | Jia Chen | Geng Wu | Geng Xu | Ding Hai |
| 2024-06-15 23:00 | Jia Chen | Geng Wu | Geng Xu | Bing Zi |
| 2024-06-16 00:00 | Jia Chen | Geng Wu | Xin Hai | Wu Zi |
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Calculate my BaZi chartFor a birth record, correct the time first
Clock 23:10 need not be Zi in true solar time. Longitude and the equation of time can move a reading back into Hai hour or across midnight. Retain the corrected date or day offset: a displayed “00:10” is ambiguous without its date. If the uncertainty in the birth record spans a boundary, calculate both sides and state the uncertainty rather than selecting a time from one remembered life event.
Why a day change affects more than one label
The day stem is the reference for Ten Gods. A different Day Master can relabel the year, month and hour stems and every hidden stem, even when those stems themselves have not changed. Compare the raw pillars before comparing labels. Historical timekeeping and modern civil time zones are different systems, so a single modern clock convention should not be described as universal traditional agreement.
Compare the same clock time
For the same civil date at 23:30, a midnight-boundary convention keeps the current day pillar, while a 23:00-boundary convention uses the next day pillar. Both place the clock time in Zi hour, but the hour stem can differ because it depends on the day stem. At 00:30 both are on the new civil day. This comparison assumes solar-time correction is off.