True solar time, and what it can move

What the correction is

Two times are involved in casting a chart: what the clock said, and what the sun standing over the birthplace said.

The gap between those two comes from two separate things. First, longitude: civil time is set by a time zone, and a time zone is a band roughly fifteen degrees wide inside which every clock reads the same, though the sun does not arrive everywhere at once. The page states how many minutes that accounts for at the birthplace entered. Second, the earth's orbit is an ellipse and its axis is tilted, so the sun runs ahead of the mean clock for part of the year and behind it for the rest — the equation of time, which the page states in minutes for the date entered.

Together those give your 真太陽時, true solar time, which is the time every Chinese almanac from the Han onward is actually built on. 《協紀辨方書》, compiled under the Qing for exactly this kind of reckoning, works in solar time throughout.

When it changes nothing

Often the correction changes nothing. The adjusted time still falls inside the same two-hour branch, so the hour pillar stands as the clock gave it, and so does every pillar above it.

That is a real result rather than an absence of one. Most English-language chart tools never perform this step at all, so they cannot tell you whether it mattered.

When it moves the hour pillar

Sometimes the correction moves the hour pillar. By the clock the birth falls inside one two-hour branch; by the sun it falls inside the neighbouring one. The hour pillar is then not the pillar a tool working from clock time alone would have given.

This is the step most English-language chart sites skip, and it is why two people can be handed different charts from the same birth certificate. It can move the hour and, close to local midnight, the day. It can never move the month or the year — those are fixed by the astronomical instant of the solar term, which happens at the same moment everywhere on earth.

When it moves the day pillar

Where a birth sits near local midnight, the correction can move the day pillar. This is the largest change the step can produce. The upper character of the day pillar is the Day Master — the single character the whole reading is built on. A tool working from clock time alone would hand back a different Day Master, and therefore a different reading in every section that follows.

Roughly one birth in twenty falls close enough to the boundary for this to happen, and the schools do not all agree on how to treat the hours either side of midnight.

What the correction can never move is the month or the year. Those are fixed by the astronomical instant of a solar term, which occurs simultaneously everywhere on earth.