Does the BaZi day start at 23:00 or at midnight?

Two schools, both old. One starts the new day at 23:00, when the 子 hour begins. The other waits for midnight. A birth between those two points gets a different day pillar depending only on which rule the calculator follows.

What actually changes

The twelve earthly branches divide the day into two-hour blocks. Eleven of them sit inside one calendar day. The twelfth, 子, runs from 23:00 to 01:00 and crosses midnight. That crossing is the whole argument.

Same date, three clock times, one calculator:

Born 15 June 1990, Hong Kong	22:30	23:30	16 June, 00:30
Day pillar	辛亥	壬子	壬子
Hour pillar	己亥	庚子	庚子

Look at the 23:30 column. Its day pillar matches the following morning, not the same evening. The day has already turned. A calculator using the midnight rule would give 辛亥 there instead, and the two charts would disagree about the day master, which is the single most consequential character in a BaZi chart.

Which is correct

Neither, in the sense you want. The 23:00 rule keeps the 子 hour intact and treats the branch as the unit. The midnight rule keeps the calendar date intact and treats the day as the unit. Both are internally consistent and both have long lineages behind them.

What matters in practice is that the rule is stated. A chart that does not tell you where it put the day boundary cannot be compared with one from another source.

If you were born between 23:00 and midnight

Your chart is convention-dependent. That is worth knowing rather than hiding. Read both, notice which one describes a life you recognise, and treat the difference as information about the method rather than about you.

Sources

• Hong Kong Observatory, Chinese calendar conversion and the sexagenary cycle

Related

• Why do two BaZi calculators give you different charts?
• Should a BaZi chart use clock time or true solar time?
• Born overseas: which time does your chart use?

See which side of the day boundary your chart falls on