The year starts at Lichun
BaZi counts years by the solar calendar. The chart year begins at Lichun (立春), when the Sun reaches 315° of ecliptic longitude. That is 3, 4 or 5 February every year, at a different time of day.
The Chinese New Year follows the Moon and falls between 21 January and 20 February. Animal horoscopes often use it. So someone born between the New Year and Lichun gets a different animal in each system.
The Chinese zodiac section of astroway.info counts the year from the New Year: there the 1990 Year of the Horse begins on 27 January 1990. In a BaZi chart the Horse year begins at Lichun, on 4 February 1990. Neither site is wrong; these are two different systems. If your birth date falls into such a gap, the chart page explains it.
Months follow the solar terms
The BaZi month is solar too. It does not start on the 1st but at one of the twelve solar terms (節), each time the Sun covers another 30°. So people born on 5 and 7 August can have different month pillars.
The day starts at 23:00
We start the day at 23:00, with the early Rat hour, as the classical tradition does. Some schools start it at midnight. For births between 00:00 and 00:59 both rules give the same day, but for 23:00 to 23:59 the day pillar differs.
If you were born between 23:00 and 23:59 by solar time, the chart page shows the second version of the day pillar and a button to recalculate with the midnight boundary. The hour pillar is the same in both versions.
True solar time
A clock shows zone time, not the time of your town. In Warsaw the Sun does not reach its highest point at 12:00 on the clock: the city lies east of its zone meridian, summer time moves the clock an hour ahead, and the season adds its own shift. BaZi counts hours by the Sun, so by default we correct for the longitude of the place and for the equation of time.
Example: Warsaw, 15 May 1990, 14:30 on the clock (summer time). True solar time is 13:57, a correction of minus 32 minutes. Sometimes the correction moves the birth to another day: Madrid, 20 July 2000, 00:30 on the clock is still 22:08 on 19 July by the Sun.
If your teacher or book uses clock time, you can switch the correction off on the chart page.
Time zone and summer time on the birth date
We do not use the time zone the town has today. For each place we take the historical zone and summer time for the birth date itself from the IANA time zone database. For example, on 1 July 1985 Kyiv kept Moscow summer time, UTC+4, not Kyiv time.
If the birth time is unknown
Then the chart has three pillars: year, month and day. We do not assume 12:00 and do not invent an hour pillar. The rest of the chart does not depend on the hour, with one exception.
If Lichun or a new solar month begins on your birth date, the year or month pillar depends on the hour. The chart is then read at noon, and the page shows the time of the term and the pillars for a birth on the other side of it.
Luck Pillars
Luck Pillars change every ten years. Their direction, forward or backward, depends on gender and the polarity of the birth year. The starting age is counted from the birth date to the next solar term when the pillars run forward, and back to the previous one when they run backward: three days equal one year. We show the starting age with months, not only whole years.
How we checked the calculation
The chart is calculated by api.astroway.info. Before every site update, 20 reference charts are checked against the live calculation. They cover the awkward cases: minutes before and after Lichun, 23:00 to 23:59 and 00:00 to 00:59 by the Sun, a date shifted by solar time, a month boundary, summer time in Warsaw, Madrid and New York, Ukraine on Moscow time, Sydney, Buenos Aires, Kashgar and a birth without a time. The expected answers were derived separately from the calculation code: JPL ephemerides, the time zone database and the cycle of 60 pairs.
The moments of all twelve solar terms for 1900 to 2100 were checked against the JPL Horizons ephemerides, and for 1901 to 2100 also against the tables of the Hong Kong Observatory. Against JPL the difference is up to 8 seconds for past years and up to 21 seconds for the forecast to 2100, because both sides predict the Earth’s rotation differently. Against the Observatory tables the dates of 2,398 of 2,400 terms match, and in the two mismatches, from 1917 and 1927, JPL confirms our date.