Roughly one client in four does not know their birth hour. Some were born at home, some hold a certificate that records only the date, and some have two conflicting accounts from two relatives.
This is a real limitation and it should be stated plainly rather than smoothed over.
What the hour actually determines
In Zi Wei Dou Shu, the birth hour anchors the placement of the twelve palaces. Move the hour and you move the whole structure: the palace that carried your wealth reading now carries your health reading. This is why an approximate hour is worse than an admitted gap — it produces a chart that reads cleanly and describes someone else.
In BaZi, the hour supplies the fourth pillar. Losing it removes information about inner life and later years, but the year, month and day pillars remain, and with them most of the elemental balance that timing analysis depends on.
What remains reliable
Without an hour, these still hold: the elemental balance of the remaining three pillars, the decade cycle sequence, annual cycle interaction, and the day pillar, which in classical practice carries the self.
What becomes unreliable: palace-specific conclusions in Zi Wei Dou Shu, and anything that depends on the hour pillar's interaction with the rest of the chart.
Reconstructing the hour
The practitioner's method is to work backwards from events you can date precisely. Not "my twenties were difficult" — that is compatible with almost any chart. Instead: the month a parent became ill, the year of a specific move, the date a business closed.
Two or three candidate hours are tested against that record. Usually one matches and the others do not. When none matches, the honest conclusion is that the hour remains unknown, and the reading proceeds on three pillars.
A practitioner who cannot say "this part is less certain" is not being confident. They are being careless.