BaZi day boundary: 23:00 or midnight?
Meridian changes the BaZi day at 23:00, when the 子 hour begins. Some calculators wait for midnight. A birth from 23:00 to midnight can therefore receive a different day pillar, so the rule must be named before two charts can be compared.
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.
Meridian changes the day at 23:00, so the 子 hour stays whole. The chart records which rule produced it.
Which is correct
This page does not treat either boundary as an official rule. The 23:00 convention keeps the 子 hour intact and treats the branch as the unit. The midnight convention keeps the calendar date intact and treats the day as the unit.
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
Ask for the stated boundary when you compare charts. A chart cast at 23:00 and one cast at midnight should be read as two calculations using different conventions.
Sources
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?
- Does daylight saving time change a BaZi hour pillar?
See which side of the day boundary your chart falls on
Worked example computed by Meridian's chart engines on 1 August 2026. The engines are deterministic, so the same input returns the same chart.
This page explains calculation conventions. It does not interpret a chart and is not advice.