Late Zi Hour 晚子时
Wǎn Zǐ Shí · Births between 23:00 and midnight, where two schools disagree about the day.
What it means
The Zi double-hour runs from 23:00 to 00:59, so it straddles midnight — the only branch that does. Births in the first half, from 23:00 to 23:59, are the "late Zi hour". One school moves the day pillar forward, treating the whole Zi hour as belonging to the next day; the other keeps the calendar day and only takes the hour stem from the following day.
Why it matters
This is a genuine, unresolved split in the tradition, not a bug. It affects roughly one birth in twenty-four, and when it applies it changes the Day Master — the single character a modern reading is built around. A site that quietly picks one convention without saying so is hiding the most consequential choice in the calculation.
Where this site uses it
The chart tool defaults to the next-day rule, which is what tyme4ts and most modern software use, and offers the same-day rule as an explicit option so you can see both charts.