Where the disagreement begins
The twelve traditional two-hour periods do not line up with a midnight-to-midnight date. At 23:30, the Zi hour has begun but the calendar date has not changed. One school changes the day at the beginning of Zi, at 23:00. Another separates late Zi from early Zi and changes the day at midnight.
Under the first convention, a 23:30 birth uses the next day’s day pillar and derives the hour stem from that next day. Under the split convention, it retains the current day pillar while its hour branch is already Zi. Both conventions have a history; a difference between them is not, by itself, an arithmetic error.
MingLiMR’s complete rule
We use split Zi: 23:00–23:59 retains the current corrected date, the hour branch is Zi, and the hour stem is derived from that same day’s stem using the Five Rats rule. After midnight the day pillar and the source day stem change together.
For example, on a Jia-Zi day, 23:30 still has a Jia-Zi day pillar and a Jia-Zi hour pillar. At 00:30 on the following Yi-Chou day, the day pillar is Yi-Chou and the hour pillar becomes Bing-Zi.
Some split-Zi practitioners retain the current day but derive the late-Zi hour stem from tomorrow. That is an additional convention; MingLiMR does not mix it into the rule above. Publishing the convention lets you compare charts meaningfully.
Solar-time correction comes first
If the recorded 23:50 becomes 00:10 the next day after the apparent-solar-time adjustment, use the next date. If 00:10 moves back to 23:50, use the previous date and the late-Zi rule. Read the solar-time guide for longitude, time zones and the equation of time.
Our earlier code once wrapped hours and minutes without carrying the date. A day-pillar error caused by that bug is different from a legitimate choice between schools. The corrected calculation carries the date as well as the time.
The bone-weight method has its own convention
The bone-weight reading uses lunar year, month, day and hour weights rather than Four Pillars. Its late-Zi convention treats birth after 23:00 as the next lunar day. Do not transfer that rule into a BaZi chart or assume that two different traditional methods must share a day boundary.
A practical comparison checklist
First establish whether the corrected time is actually between 23:00 and 23:59. At 00:30, both main BaZi schools already use the new date; disagreement then comes from another input or convention.
Next compare the other calculator’s day boundary and hour-stem rule. Finally check whether the solar adjustment crosses midnight and whether the original civil time was resolved with the right historical time zone.
If the time is only a guess, do not turn it into a precise 23:30 entry simply to obtain a full chart. A three-pillar chart with explicit limits is more honest than a confident chart built on an invented hour.
Sources and reading notes
The explanations here are our own English wording. Classical titles and Chinese symbols identify the source tradition; they do not imply agreement between all schools.
MingLiMR’s declared late-Zi-hour convention
This guide explains the product’s midnight day boundary and date rollover. Competing 23:00 conventions exist; the chosen rule is not presented as universal consensus.