True Solar Time — how many minutes is your birthplace off?

True Solar Time (眞太陽時) reads the hour by where the sun actually stood over your birthplace. Korean standard time is set to the 135°E meridian, but Seoul sits at 126.98°E — so the clock runs about 32 minutes ahead of the sun. The formula is (standard meridian − longitude) × 4 minutes: one degree of longitude is exactly four minutes. In Saju and BaZi, the hour pillar (時柱) is built from this solar time, not the clock — so when the correction crosses a branch boundary, your hour pillar changes. Compute your birth city's correction below.

Your birthplace correction calculator

Uses the same formula as the Gwiraedang chart engine (automated tests lock its output to the engine's). Add your birth time to see whether it sits near an hour-branch boundary.

Birth cityBirth time (optional — checks the branch boundary)

Correction about −32 min= (standard meridian 135° − longitude 126.98°) × 4 min

Born in Seoul, your true solar time runs about −32 min against the clock — the sun crosses your local meridian after clock noon. If that shift crosses a two-hour branch boundary, your hour pillar changes.

This calculator uses the same formula as the Gwiraedang chart engine (1° of longitude = 4 minutes), and automated golden tests lock its output to the engine's. The equation of time (a seasonal ±16-minute wobble) is deliberately not applied, matching common manseryeok practice — see the convention note below.

Why the clock and the sun disagree

Standard time is a convention: a whole country keeps its clocks to a single meridian. Korea and Japan both use 135°E (UTC+9), so the moment the sun actually crosses your local meridian drifts later the farther west you are — Seoul (126.98°E) about 32 minutes, Busan (129.08°E) about 24 minutes after clock noon. East of the meridian it flips: Tokyo (139.69°E, against Japan's 135°E standard) runs about 19 minutes ahead of the clock. For births outside Korea, the same formula applies against that country's own standard meridian (standard UTC offset × 15°).

Why it matters — a birth where the hour pillar actually splits

The hour pillar is judged by two-hour branches on True Solar Time. Corrections run around 30 minutes, so a birth near a branch boundary (odd o'clock sharp — 09:00, 11:00…) shifts a full branch on the correction alone. A real case — Mar 3, 1998 · 11:05 · Seoul · F: Seoul's ~−32 minute correction turns 11:05 into 10:33 solar time, and the hour pillar is 己巳 기사; computed from raw clock time instead, it lands on 庚午 경오. Those stem–branch pairs are not hardcoded — they are what the Gwiraedang engine actually computed when this page was built, locked by the golden tests behind the public benchmark.

Away from a boundary, the correction changes nothing: the benchmark's Incheon 1994 case stays in the Sin (申) hour with or without it. The correction doesn't always change the hour pillar — only when the birth sits near a boundary. That's why you need both numbers: how many minutes your birthplace is off, and how far your birth time sits from the nearest boundary.

A note on conventions — different ≠ wrong

Plenty of calculators build the hour pillar from raw clock time, with no True Solar Time at all. When such a tool disagrees with ours on the hour pillar, that's not a calculation error — it's a convention difference, and different is not the same as wrong. What matters is whether a tool tells you which corrections it applied. Gwiraedang discloses every applied correction — longitude, minutes, daylight saving — right on the result screen.

Two more disclosures. First, we don't apply the equation of time (an astronomical wobble that swings solar time by up to about ±16 minutes across the seasons) — a deliberate choice to match common manseryeok practice, which makes Gwiraedang's True Solar Time precisely a “longitude-based (mean solar time) correction.” Second, from March 1954 to August 1961 Korean standard time was UTC+8:30 — 30 minutes off today's. For births in that era we follow the common manseryeok convention of computing against the 135° meridian, and flag the possible 30-minute branch-boundary gap on the result screen.

What is True Solar Time, and why is it different from clock time?

True Solar Time reads the hour by where the sun actually stood over your birthplace. The clock time we use is an agreed convention — one standard meridian for a whole country (135°E for Korea) — so the farther you are from that meridian, the more the clock drifts from the real sun. The drift is 4 minutes per degree of longitude: Seoul sits at 126.98°E, so (135 − 126.98) × 4 ≈ 32 minutes — the clock runs about 32 minutes ahead of the sun.

Does the True Solar Time correction always change the hour pillar?

No. Each hour branch is a two-hour block, so the correction only changes your hour pillar when it carries the time across a block boundary — boundaries fall at the top of every odd-numbered hour (09:00, 11:00, and so on). A birth in Seoul on March 3, 1998 at 11:05 becomes 10:33 in solar time and crosses back over the 11:00 boundary into the Sa (巳) hour — but a birth in Incheon on January 17, 1994 at 15:41 becomes 15:08, which stays in the Sin (申) hour either way. Enter your birth time in the calculator above to see whether yours sits near a boundary.

My hour pillar is different in another Saju or BaZi calculator — which one is right?

That's usually a convention difference, not a calculation error — different is not the same as wrong. Some tools build the hour pillar from raw clock time, some apply the longitude correction, and some schools add the equation of time on top. Gwiraedang applies the longitude-based True Solar Time correction (the equation of time is deliberately left out, to match common manseryeok practice) and discloses exactly which corrections were applied right on the result screen. What to look for in any tool is whether it tells you which convention it used.

See your chart with the correction applied

Free, no sign-up. True Solar Time, daylight saving and overseas births handled automatically — every correction disclosed.

Free Saju calculator