Korea's Timezone History

Korea changed its standard meridian four times and ran daylight saving in twelve separate stretches.

Korea kept local time until 1908, then UTC+8:30, then UTC+9 from 1912, then back to UTC+8:30 between 1954 and 1961, then UTC+9 again.

Daylight saving ran in 1948–51, 1955–60, and again around the Seoul Olympics in 1987–88. A birth inside one of those windows was recorded on a clock running an hour fast.

Any calculator that assumes today's UTC+9 for a 1955 or a 1988 summer birth is out by thirty to sixty minutes, which is enough to move the hour pillar a full branch — and for a late-evening birth, the day pillar with it.