True Solar Time

Time measured by where the sun actually is, rather than by the clock on the wall.

Clocks run on zones; the sun does not. Korea keeps UTC+9, which is centred on 135°E, but Seoul sits at about 127°E — so Seoul's sun reaches noon roughly 32 minutes after Seoul's clocks say it has.

On top of that, the earth's orbit is elliptical and its axis is tilted, so apparent solar time drifts up to about 16 minutes ahead of or behind mean time depending on the date. That correction is called the equation of time.

Both corrections have to be applied before the day and hour pillars can be read. A calculator that skips them is producing a chart from the wrong moment — usually harmlessly, but not for anyone born within half an hour of a boundary.