Timezone means historical clock rules
Choose the regional timezone for the birth location, rather than a present-day fixed offset. A backward clock change can repeat a local time; a forward change can omit it. Repeated times represent distinct instants.
Day boundary is a convention
A civil date usually changes at midnight; some chart conventions change at the start of Zi, 23:00. The option can change the day pillar. Civil and apparent-solar clocks also differ; longitude and corrections are required for the latter.
When the hour is unknown
Keep an unknown hour unknown. Three known pillars do not establish an hour-dependent result. Ziwei star placement needs an hour; a default midday value is not evidence. An estimate only supports a chart under that assumption.
Sources and further reading
Explanations are written by this site; original excerpts are labeled separately. See the source catalogue for editions and scope.
Reviewed: 2026-10-09