Clock time is not solar time
A birth time copied from a civil register is a legal time: the one the clocks of the country showed that day. It is not the real position of the Sun above the place.
Two gaps stack up. The first comes from the time zone itself, which covers a wide band of longitudes: within one zone the Sun does not reach its highest point at the same instant in the west and in the east. The second comes from legal time, which drifts further still from the zone according to the administrative decisions in force.
None of this is a detail for the Four Pillars. The hour pillar is determined on a two hour slice. A large enough cumulative gap tips a birth from one slice into the next, and therefore changes one stem and one branch out of eight characters. This is why the correction is made before everything else, and not after the fact.
Four minutes per degree of longitude
The correction itself is arithmetical. The longitude of the birth place is compared with that of the meridian of its zone, and four minutes are counted per degree of longitude. West of the reference meridian the Sun is behind the clock; east of it, the Sun is ahead.
That is exactly what the engine applies before setting the hour pillar. The time used for the calculation is therefore never the raw time entered: it is the time brought back to the Sun of the place.
One consequence is worth noting. Two people born at the same clock minute, in the same country and the same time zone, but several hundred kilometres apart on the east west axis, may not receive the same hour pillar. The birth place is not one more formality in the form, it enters into the calculation.
The day starts at 11pm, with the Zi double hour
The twelve double hours do not begin at midnight. They begin at Zi, which runs from 11pm to 1am, and then follow one another two hours at a time.
The result is a rule that often comes as a surprise: the BaZi day starts at 11pm, not at midnight. A birth at 11.30pm already belongs to the following day, with the day pillar that goes with it, and therefore a different Day Master. This is one of the most frequent errors in charts cast by hand, and it does not affect only the hour pillar: it also shifts the day pillar, which is the pivot of the whole reading.
The year switches at Lichun, not at Chinese New Year
The year pillar changes neither on 1 January nor at Chinese New Year. It switches at Lichun, around 4 February.
The consequence is direct and often unwelcome: a birth in January belongs to the previous animal year. Someone who has always believed themselves to be one animal, because they read their birth year in a table, may discover that their year pillar carries the animal before it.
This is worth saying plainly, all the more so because that year pillar is precisely the one the annual horoscope isolates. In a complete chart the year animal is only one character out of eight, and its determination obeys Lichun like everything else.
The month switches at the Jie Qi
The month pillar follows the same solar logic. It changes at the twelve major solar terms, the Jie Qi, and not on the first day of the civil month. A chart built on ordinary months gets the month pillar wrong every time a birth falls between the start of the civil month and the matching solar term.
These two markers meet at one point. Lichun opens both the year and the first BaZi month, the February one, which carries the branch Yin, that of the Tiger. The division of the year into twelve BaZi months is therefore not a tracing of the Gregorian calendar shifted by a few days: it is another division, indexed on the course of the Sun.
The month pillar is also the one the sequence of Da Yun decades starts from. An error on a switch therefore costs not one character, but one character and the entire decade series that follows from it. Each of these pillars, year, month, day and hour, remains a point of the sexagenary cycle.
What this precision is worth, and what it is not
It has to be said honestly where the precision stops. The switches used here, Lichun and the Jie Qi, are set to the day. That is amply enough in the vast majority of cases: a few days away from a switch, the pillar is the same whatever the exact minute of the solar term.
The reservation concerns births sitting right on a boundary, a 3, a 4 or a 5 February for instance, or the very day of a solar term. There, only a fine determination of the term settles the matter, and a chart cast with day level precision may point to the wrong pillar.
The good practice is therefore simple. Enter the birth place and time with as much care as possible, let the longitude correction do its work, and when the date falls on a known boundary treat the pillar concerned as needing checking rather than as settled.
