Sijin (時辰): What Actually Changes Every Two Hours
Twelve branches a day, one rule, and one hour that belongs to tomorrow before the date does.
Bokki Editorial
The short answer
Your Saju chart never changes, and your day pillar changes once every calendar day. One layer below that is sijin (時辰) — the two-hour branch, and it is the one part of a reading that turns over while you're looking at it. Some apps handle this by splitting the day into three fixed tabs — morning, afternoon, night — and calling it done. Bokki computes it instead. Each two-hour branch gets its own hour-stem, worked out from today's own day-stem using a rule called the Five Rats Escape (五鼠遁), the same method Bokki's engine already uses to build the hour pillar of a birth chart. That stem, read against your day master, names which of five energies these particular two hours light up for you — and because the stem is actually computed, not merely looked up from a fixed table of twelve, tomorrow's same two-hour branch will not automatically repeat today's answer.
The rule — Five Rats Escape, and one hour that runs ahead of the date
The twelve two-hour branches — 子 23:00-01:00, 丑 01:00-03:00, and on through 亥 21:00-23:00 — are fixed clock windows and never move. What moves is which of the ten heavenly stems sits over each branch, and that depends entirely on today's own day-stem: five pairs of day-stems (甲己, 乙庚, 丙辛, 丁壬, 戊癸) each anchor a different starting stem for the 子 hour, and every branch after that simply advances the stem by one. That is the whole rule — no lookup table of 144 hour combinations, just one starting point and a count. There is exactly one wrinkle, and it is old enough to have its own name: 夜子時, the "night zi-hour." Cross-checked directly against lunar-typescript — the independent, traditional engine Bokki already uses to build a birth chart's hour pillar — the 子 branch that runs from 23:00 to midnight already uses tomorrow's day-stem as its anchor, even though the calendar date itself, and the day pillar shown elsewhere in a reading, do not roll over until midnight. The last two hours of a day belong, quietly, to the next day's rhythm before the date does.
Worked example — one 子 hour, two different stems, same date
Take one real chart's day master — Water, the same person Bokki's piece on the hour-pillar boundary already used. On 25 August 2026, a day whose own stem is 辛 (day pillar 辛未), running the rule across all twelve branches gives a hour-stem that steps forward by one every branch: 子戊, 丑己, 寅庚, 卯辛, 辰壬, 巳癸, 午甲, 未乙, 申丙, 酉丁, 戌戊, 亥己 — Earth appears at 子 and 丑, then Metal at 寅 and 卯, then Water, Wood, Fire, and back to Earth at 戌 and 亥, each element getting exactly two branches, four hours, before handing off to the next. Now look at 24 August specifically, whose own stem is 庚 (day pillar 庚午). At 00:00, the very first 子 hour of that date, the rule anchors on 24 August's own stem and gives 丙子 — Fire, lighting up this day master's wealth energy (財星). At 23:00, the last 子 hour still falling on the calendar date 24 August, the rule has already switched to anchor on 25 August's stem instead, giving 戊子 — Earth, duty energy (官星) — a genuinely different two-hour reading, on the same date, in the same 子 branch, an hour before midnight ever arrives.
What this line doesn't read
This calculation does not correct for true solar time, the way an hour-of-birth pillar does. That correction needs to know where the sun actually stood above a specific place at a specific moment, which means knowing the user's current location — and Bokki does not request GPS access, and should not. So this reading uses the device's own civil clock, already adjusted for its time zone, exactly as shown — an honest limit, not an oversight. It does not read your birth hour at all; your day master, the one input this calculation needs from your permanent chart, was already fixed the moment your chart was built, and today's hour-stem is computed fresh from today's day-stem, never from the hour you were born. It does not carry over any judgment from a different card on the same screen — whether today itself is favorable, whether yesterday differed from today, or how strong your day master runs are all separate axes with their own explanations. And it never frames a two-hour window as a deadline, a warning, or something to chase before it closes; it names what these two hours are, in plain language, without urgency built in.
Closing note — twelve branches, one starting point, one hour ahead of the date
In short: sijin computes a genuine hour-stem for whichever two-hour branch you're in right now, anchored on today's own day-stem and advancing by one stem per branch — Earth twice, then Metal twice, then Water, Wood and Fire, each taking its own pair of branches before the cycle returns to Earth. The one deliberate exception is the last 子 hour of the day: cross-checked against the same independent engine Bokki already trusts for a birth chart's hour pillar, that hour anchors on tomorrow's day-stem rather than today's, so the calendar date and the astrological "day" it belongs to genuinely part ways for sixty minutes. On 24 August 2026 that meant Fire and wealth energy at the stroke of midnight, and Earth and duty energy an hour before the next midnight — two different readings, one date, one branch, and a rule old enough that Bokki's engine had to go looking for it before it could claim to compute this two-hour window honestly.