How we compute your chart

You enter a solar (Western calendar) date, an hour, your sex, and — optionally — a birthplace. If the birthplace is known, the clock hour is corrected to local solar time first. The corrected moment is then converted to the Chinese lunar calendar and handed to iztro, which places the palaces and stars, and to lunar-javascript, which writes the four pillars. Every Chinese star, palace and branch name comes back as its stable Chinese key, and our own dictionary supplies the English — we never show a library’s built-in translation.

Applied · mean solar time, when both the hour and the birthplace are known

A destiny chart is timed by the sun, not the clock. Civil clocks run on a time zone’s standard meridian, so a birth away from that meridian happened at a different solar time than the clock said. We shift the clock reading by longitude × 4 minutes − standard UTC offset × 60. For a birth in Ürümqi (87.6°E, on Beijing time) that is about −2 hours; for Seoul (126.98°E, UTC+9) about −32 minutes; for London on the meridian, nothing. The shift can move the birth across an hour boundary — and so change the hour branch and, with it, every palace — or across midnight, in which case the calendar date moves too.

Not applied · the equation of time

We apply mean solar time. The true sun runs up to about 16 minutes ahead of or behind mean time across the year (the equation of time), and we do not add that correction. For most births it changes nothing; within a few minutes of an hour boundary it could.

Not applied · enter your standard-time hour

We do not model daylight saving. The form says it plainly: “if you were born during daylight saving time your clock was one hour ahead — subtract one hour for precision.” That subtraction is yours to make; the engine treats the hour you enter as standard time.

Not applied · historical time-zone changes

The offset used is the birthplace’s currentstandard offset from our city table. Places that have changed zones — mainland China’s five zones before 1949, Singapore’s several shifts, and others — are not modelled historically. If you know your birthplace kept a different offset at the time, enter the longitude and that offset by hand.

Applied · split into early and late Rat on the Zi Wei chart

The Rat hour straddles midnight: 23:00 to 00:59. Schools differ on how to treat it. For the Zi Wei Dou Shu chart we follow iztro’s split: a birth at 23:00 to 23:59 is the late Zi hour (晚子時) and sits on that day; 00:00 to 00:59 is the early Zi hour (早子時) and sits on the next.

Applied · the BaZi day pillar stays on the same calendar day for 23:00–23:59

For the BaZi pillars, a birth between 23:00 and 23:59 keeps that calendar day’s day pillar with the Zi hour pillar — the convention most modern practitioners use. Some schools roll the day pillar forward to the next day at 23:00. We do not, and if your reading elsewhere did, that is the difference.

Applied · exact conversion; the first half of a leap month counts as that month, the second half as the next

You enter a solar date, so a leap-month birth needs nothing special from you: the conversion to the lunar calendar is exact. Where it matters is placing the Life palace, which is counted from the lunar month. iztro is called with its leap-month fix on: a birth in the first half of a leap month is counted as belonging to that month, a birth in the second half to the following one — the mainstream convention.

Applied · cast at noon, flagged everywhere, hour pillar omitted

Without an hour the chart is cast at noon (the Horse hour) and every page that depends on it says so. No solar-time correction is applied, since there is no hour to correct. The BaZi side shows three pillars — year, month, day — and leaves the hour pillar out rather than invent one.

Not applied · not distinguished

Two people born in the same hour, in the same place, of the same sex get the same chart. The form takes hours, not minutes, so twins cannot be told apart even in principle. Traditions that distinguish twins by minute, by order of birth, or by reading one twin’s chart from the other’s opposite palace are not implemented.

The short version: give us the hour and the birthplace, entered in standard time, and the chart is computed the way a careful practitioner would compute it by hand. Where we simplify, this page says so, and the pages that rest on an assumption say so too.