After 23:00: Zi Hour begins without changing the day

Fifteen minutes past the previous case, at 23:15, the hour pillar has changed to Jia-Zi while the day pillar has not moved. This is the convention doing exactly what it claims: Zi Hour opens at 23:00, and the day waits for midnight.

The pair is the cleanest demonstration of a rule that is often described imprecisely. Readers told that Zi Hour is the start of the new day reasonably expect both to change together. Under this convention they do not, and the fifteen-minute gap between these two cases is where that becomes visible.

If your birth falls in this window, the practical consequence is that your hour pillar and your day pillar come from different rules. That is unusual enough that some calculators will silently disagree with this one, and knowing which convention each uses is the only way to tell which is which.

The wider point is that a rule can be correct and still be badly summarised. Most of the confusion around midnight births comes from short explanations rather than from disputed tradition. When you read any guide to this boundary, including this one, the useful test is whether it separates the hour boundary from the day boundary explicitly. If it does not, it cannot tell you what happens between them.