Before 23:00: the chart remains in Hai Hour

This opens a set of four cases about the most argued-about boundary in Bazi: when the day changes and when Zi Hour begins. The first case sits at 22:45 true solar time, comfortably before the 23:00 mark, and returns a Gui-Hai hour pillar with the day unchanged.

The reason this is contested is that 23:00 and 00:00 are both defensible places to draw the line, and different schools draw it differently. One convention treats the day as changing at 23:00, so that someone born at 23:30 belongs to the next day. Another keeps the day until midnight and changes only the hour branch.

This site uses the second convention, and states it. The four cases in this category exist so that the choice is visible and testable rather than buried in a method page. A reader who compares this calculator against another and finds a differing day pillar for a late-night birth has found this disagreement, not a bug.

Because the disagreement is about a convention rather than a calculation, no amount of checking the arithmetic will resolve it. The only productive question is which convention you intend to use, and then whether the tool applies that one consistently. That is a different kind of question from the ones the rest of this suite answers, and it is worth being clear that the two do not mix.