Korea moved its clocks six times. Your chart probably ignores it

Saju is calculated from true solar time — where the sun actually was — not from the clock in the delivery room. Getting from one to the other requires knowing what rule that clock was following, and Korea has changed the rule more often than almost anywhere.

The meridian moved four times

Korea kept local mean time until 1908. Then UTC+8:30, centred on 127.5°E. Then UTC+9, centred on 135°E, from 1912. Then back to UTC+8:30 between 1954 and 1961. Then UTC+9 again, where it has stayed.

The practical consequence is a half-hour swing. Seoul sits at about 127°E, so under today's UTC+9 its clocks run roughly 32 minutes ahead of its sun. Under the 1954–61 rule, they ran almost exactly with it — the correction is about two minutes.

A calculator that assumes today's UTC+9 for a 1957 birth is therefore applying a half-hour correction that should not be there.

And daylight saving ran twelve times

Less well known: Korea observed DST in 1948–51, again through 1955–60, and again in 1987 and 1988 around the Seoul Olympics.

A birth inside one of those windows was recorded on a clock running a full hour fast. Ignore it and the whole chart shifts an hour.

Why an hour is not a rounding error

The hour pillar is one of your four. Its branches are two hours wide, so an hour of error puts you within a coin-flip of the wrong branch — and if the birth was in the evening, it can push you across the 23:00 boundary where the day pillar rolls over.

That last case is the serious one. The day pillar's upper character is your day master, the character representing you, against which every other judgement is measured. Get it wrong and you have not made a small error; you have read someone else's chart.

What to ask

Any tool worth trusting should be able to tell you the UTC offset it used and whether daylight saving applied. If it cannot, and you were born in a Korean summer between 1948 and 1988, treat the chart as unverified.