What are 절기 (the 24 solar terms)?
Where a Saju month actually begins — and it is never the 1st.
In plain words
The calendar on your wall counts days. The old East Asian calendar also counted where the sun is. Draw the sun's yearly path as a circle, cut it into twenty-four equal slices of fifteen degrees each, and mark the point where each slice begins. Those twenty-four marks are the 절기 — the solar terms. They are why 입춘, 하지 and 동지 fall on almost the same date every year but never on exactly the same minute.
The twenty-four alternate: twelve of them are 節, the sectional terms that open a month, and twelve are 中氣, the mid-terms that sit in the middle of one. For Saju only the twelve 節 matter, because those are the doors. When 입추 arrives, the Saju month changes — even if it is the seventh of August and even if it arrives at 19:42 on the almanac's UTC+8 clock, which is 20:42 in Seoul.
How Bokki computes it
Bokki builds the month pillar from a 만세력 perpetual calendar, so the boundary is the 節, never the 1st. In 2026 those twelve doors are 소한 5 Jan 16:23, 입춘 4 Feb 04:02, 경칩 5 Mar 21:59, 청명 5 Apr 02:40, 입하 5 May 19:48, 망종 5 Jun 23:48, 소서 7 Jul 09:56, 입추 7 Aug 19:42, 백로 7 Sep 22:41, 한로 8 Oct 14:29, 입동 7 Nov 17:52 and 대설 7 Dec 10:52. Every one of those has a minute on it, and Bokki uses the minute. Those minutes are UTC+8 — the clock the almanac library reports every solar term in — so on a Korean wall clock each one falls an hour later: 입춘 2026 at 05:02, not 04:02.
Here is what that means in practice, run through the actual code. A birth in Seoul on 7 August 2026 at 19:00 gets the month pillar 乙未. The same date at 21:00 gets 丙申. Same day, different month pillar, because 입추 landed between them — at 19:42:43 on the almanac's UTC+8 clock, which is 20:42:43 on a Seoul one. That last step is the part to watch: give Bokki a birthplace and it reads that single instant on that city's own clock, so the first 丙申 minute in Seoul is 20:43, not 19:43. Leave the birthplace blank and the minute you typed is measured against the UTC+8 table itself, where 19:43 is already 丙申. This is the single most common way an online Saju calculator goes wrong: it turns the month on the 1st, or it turns it on the date of the term rather than the moment.
The year works the same way and turns at 입춘, the term at 315 degrees of solar longitude. Measured: 입춘 fell at 23:18 on 4 Feb 1984, at 05:28 on 4 Feb 2018, at 22:58 on 3 Feb 2021, at 16:27 on 4 Feb 2024, at 22:10 on 3 Feb 2025 and at 04:02 on 4 Feb 2026 — all of them on the almanac's UTC+8 clock, an hour behind Korea's. So someone born on 3 February 2001 at 23:50 in Seoul carries the year pillar 庚辰, not 辛巳 — 입춘 2001 did not arrive until 02:28 on the 4th, which is 03:28 on a Seoul clock. Three of Bokki's calculations read that same moment: the Saju year pillar, the Nine Star Ki year and the Feng Shui year. One ruler, so they cannot disagree about which year you were born into. The moment itself comes from the almanac; which minute of your own clock it lands on depends on the birthplace you give it, which is what the true solar time piece spells out.
What we do not compute
We do not derive the term moments ourselves. Bokki reads them from the perpetual-calendar library it uses; we did not write that ephemeris and we do not claim its accuracy as our own work. What we did do is check that every screen reads the same table, so that the home card, the month list and the chart never point at different months.
And those moments are stated on the East Asian standard meridian, while your birth is corrected to true solar time at the place you were born. Those are not the same clock. If you were born within about an hour of a boundary, another 만세력 can legitimately land on the other side of it, and your month pillar — or your year pillar — may differ. We would rather tell you that than let a boundary look sharper than it is. If that is your situation, check a second source before leaning on the reading.
We also do not do anything with the twelve 中氣 beyond letting them sit in the table. They matter for the lunisolar calendar's leap months and for farming almanacs, but they are not month boundaries in Saju, so Bokki does not read them into a chart.
Closing note — twelve doors, each one timed to the minute
In short: a Saju month turns at one of twelve 節, never on the 1st, and a Saju year turns at 입춘 — a moment, not a date. The 2026 table above puts 입추 at 19:42 on 7 August on the almanac's UTC+8 clock, which is 20:42 in Seoul, so a Seoul birth at 19:00 that day gets 乙未 and one at 21:00 gets 丙申; the same logic put 입춘 2001 at 02:28 on 4 February — 03:28 in Seoul — which is why a birth at 23:50 on 3 February 2001 carries the year pillar 庚辰, not 辛巳.
We read those minute-level moments from the perpetual-calendar library rather than computing them ourselves, but we did make sure the Saju year, the Nine Star Ki year and the Feng Shui year all read the same 입춘 moment — one ruler, so the three calculations cannot disagree about which year you were born into. What we do not do is touch the twelve 中氣 for anything beyond letting them sit in the table; they set lunar leap months, not Saju boundaries.