What is 진태양시 (true solar time)?
Noon on the clock is not noon in the sky — and the hour pillar follows the sky.
In plain words
Noon means the sun is at its highest. But clocks do not work that way — a whole country shares one time, set to a single line of longitude, so almost nobody's noon is really noon. Korea sets its clocks by 135 degrees east, which runs through the sea past Japan. Seoul sits at about 127 degrees. The sun over Seoul reaches its highest point a good half hour after the clock says twelve.
The hour pillar of a Saju chart divides the day into twelve two-hour branches, and those branches are anchored to the sun, not to the clock. So if we took your birth certificate time at face value we would sometimes put you in the wrong branch entirely — and the hour pillar is a quarter of your chart. That is what true solar time fixes.
How Bokki computes it
Two corrections, added together. The first is longitude. Your time zone's meridian is fifteen degrees times its offset from UTC, and the earth turns one degree every four minutes. Seoul at 126.98 degrees against a 135-degree meridian gives minus 32.1 minutes. Busan, further east at 129.08, gives minus 23.7. Tokyo at 139.69 gives plus 18.8. Madrid gives minus 74.8, because Spain keeps central European time while sitting west of Greenwich. Los Angeles gives plus 7.0.
The second is the equation of time. The earth's orbit is an ellipse and its axis is tilted, so the sun runs a little fast in some seasons and a little slow in others. Across a year our approximation swings from about minus 14.6 minutes in mid-February to about plus 16.4 minutes at the start of November. Add the two together and Seoul comes out around minus 41 minutes in mid-January and around minus 38 in mid-July. If the shift pushes past midnight, the date rolls with it.
Here is what it changes, run through the actual code. A birth in Seoul on 20 March 1995 at 11:20 becomes 10:40 in true solar time, and the hour pillar moves from 壬午 to 辛巳. At 23:15 it becomes 22:35 and moves from 戊子 to 丁亥. At 07:10 it becomes 06:30 and moves from 庚辰 to 己卯. On 4 June 1988 at 13:05 it becomes 12:35 and moves from 癸未 to 壬午. Korea is a country where this bites often, because the whole peninsula sits five to eight degrees west of the meridian it keeps time by.
Bokki applies this only when you give both a birth time and a birthplace. There are 44 places in the table. If you do not choose one, we use your clock time as given rather than guessing your longitude — a wrong correction is worse than none.
What the clock did, and what we still do not compute
The clock itself did not always keep standard time, and we take that back out before correcting. Korea ran summer time, an hour ahead, in 1948–51, 1955–60 and 1987–88, and from 1954 to 1961 its clocks ran on 127.5 degrees east, half an hour behind today's; many other countries still move their clocks every year. We do not keep that history ourselves. Each place in the table carries its time-zone name, and for your date and clock time we ask the IANA time-zone database — the one browsers and servers already ship — which offset that clock was actually running on. A Seoul birth at 08:30 on 15 July 1987 was on summer time, so it becomes 06:52 apparent solar time and falls in 卯; read on the standard offset alone it would be 07:52 and 辰.
Two edges remain. Hanoi is the one place read on a single standard offset: the database has no zone of Hanoi's own, and the zone it uses for Vietnam carries South Vietnam's UTC+8 of 1959–75, which Hanoi's clocks never ran. And a clock change leaves one hour that cannot be read cleanly. When clocks were set back, every time in that hour happened twice, and we read it as the later, standard-time one; when they jumped forward, the times in the skipped hour never happened, and we read them on the offset from just before the jump. If you were born in such an hour, treat the hour pillar as uncertain.
And the solar terms themselves are kept on a different clock than yours. The almanac library we build charts from reports every one of the 24 terms on UTC+8, Chinese standard time — we checked all 91 years from 1950 to 2040 against an independent astronomical calculation, and the table matches UTC+8 to within a minute every year and misses UTC+9 by a full hour every year. A solar term is one instant for the whole world, so when you choose a birthplace we read the year and month pillars at the actual moment of birth: your clock time, minus the offset that clock was running on that day, carried onto the table's UTC+8. On 4 February 2026 a Seoul or Tokyo birth enters the new year at 05:03 and a Beijing birth at 04:03 — the same instant, read on each city's own clock. The day and hour pillars stay on apparent solar time at the birthplace. Without a birthplace we do not know your clock's offset, so the time you type is compared against the table exactly as typed, and a Korean clock time turns the year an hour early — at 04:03 instead of 05:03. If you were born within an hour of a solar term, choose your birthplace, or treat that pillar as uncertain and check it against a local almanac.
The equation of time we use is an approximation, good to well under a minute, not an ephemeris lookup. And where the day itself turns is a place tradition genuinely splits: in the calendar we use, the day pillar changes at the sun's midnight while the hour pillar enters the 子 branch at 23:00 by the clock and takes its stem from the next day. A reader who follows the other convention will read an 11-o'clock-at-night birth differently, and neither of you is making it up.
Closing note — two corrections, added, and where they stop
To sum up: true solar time is longitude minus the zone meridian (Seoul −32.1 minutes, Tokyo +18.8, Madrid −74.8) plus the equation of time (roughly −14.6 to +16.4 minutes across a year), applied only when both a birth time and a birthplace are given. In Seoul that swings the hour pillar of a real birth — 11:20 to 10:40, 壬午 to 辛巳 — across a two-hour branch boundary that a clock-time reading would have missed entirely.
The correction stops where the input does: the clock's own offset that day — summer time, Korea's 1950s meridian of 127.5°E — comes from the IANA time-zone database rather than from us, with Hanoi left on its standard offset and the doubled or skipped hour of a clock change read one fixed way; the equation of time is an approximation rather than an ephemeris lookup; and the almanac's 24 solar terms are kept on UTC+8 (verified across 1950–2040), which we reconcile with your clock only when a birthplace tells us its offset — without one, a birth within an hour of a term can land on the wrong side of a year or month boundary. And the boundary itself is a place tradition splits — our calendar turns the day at midnight but the hour pillar at 23:00, taking its stem from the next day, and a reader following the other convention would read an 11pm birth differently. Neither of you would be making it up.