How to calibrate a chart when the birth time is uncertain
Half an hour can change the hour pillar in BaZi or the Life palace in Zi Wei. This page explains how to judge how far your birth time can be trusted, which tier of the chart it affects, and how to check with two adjacent charts.
Start with where the time comes from
A birth certificate, a hospital record, a mother's memory and family lore are trustworthy in that order. A certificate records clock time, usually the standard time of the hospital's location; family lore is often a period ("just before dawn", "around dinner") rather than a moment.
Write the source down first, then decide whether this chart deserves trust down to the hour pillar. If the source is a period, cast one chart at each end of it and see whether the conclusions change.
Three common traps
Daylight saving time. China observed it in the summers of 1986–1991; the United States, Europe and Australia still spend half of every year on it. The site subtracts it automatically, provided the time zone you enter is right.
Longitude. BaZi is cast in true solar time, and the same clock hour differs by more than two hours between Ürümqi and Shanghai. With a birthplace entered, the site corrects for longitude and the equation of time; without one it can only use the zone's central meridian.
The late Zi hour. For births between 23:00 and 23:59, the BaZi engine and the libraries used for Zi Wei and Qi Men disagree on whether the day changes. The site freezes that disagreement in the first line of the result and shows both conventions instead of choosing for you.
Checking with two charts
Use the calendar tool to see how far your birth moment sits from the nearest hour boundary and solar-term boundary. Within twenty minutes of a boundary, cast one chart on each side.
Take the item that differs most between the two charts (usually the hour pillar in BaZi, the Life palace in Zi Wei) and test it against two or three events that have already happened with known years: school, a move, a marriage. Use the chart whose stages fit first, but keep in mind that it might be the other one.
Should daylight saving time be subtracted from the certificate time?
Only if that place was on daylight saving time that day. The site decides from the time zone you enter and subtracts it; the result states how many minutes were removed.
Can a chart be cast from an approximate period?
Yes, but trust it only down to the day pillar. Hour-dependent items such as the hour pillar or the Life palace should be compared across charts cast at both ends of the period.
Why do the BaZi page and the calendar page show different hour pillars?
The BaZi page uses true solar time (corrected for longitude and the equation of time); the calendar page uses standard clock time. Both pages state their convention.