What is true solar time?
True solar time takes noon as the moment the sun is actually highest where you are, not what the clock says. Saju follows the sun, so your birth time is converted to true solar time first.
Why it differs from the clock
Korea's time zone is set for 135°E. Seoul sits near 127°E, so the sun peaks there about 32 minutes later. Each degree of longitude is four minutes.
It shifts through the year
Earth's orbit is not a perfect circle and its axis is tilted, so the sun's highest point drifts up to about a quarter of an hour either way during the year. This is called the equation of time.
Daylight saving and old time zones
Korea used daylight saving time in 1948–1951, 1955–1960 and 1987–1988, and from 1954 to 1961 its standard time ran 30 minutes behind today's. Births in those years need extra care.
An example
Born at 12:40 on July 4, 1995, in Seoul: subtract 32 minutes for longitude and 4 for that day's equation of time, and the true solar time is 12:03. Saju hours come in two-hour blocks, so half an hour can change the hour pillar.
Try it with your chart