The 23:00 Rule

The Rat hour runs from 23:00 to 01:00 and belongs to the following day. A birth at 23:30 therefore carries the next day's day pillar, not the current one.

Some schools split the Rat hour instead, keeping 23:00–00:00 on the old day. It is a genuine doctrinal disagreement rather than an error, and it affects only births in that one hour — but for those births it changes the day master, which is the most important character in the chart.