Sukuyo leap month: what changes in the calculation?
A Sukuyo leap month changes the lunar-date record, not the 27-mansion roster. Keep the repeated month number, leap flag, and day for lookup.
Overview
Retain the leap marker and numbered month before consulting the twelve-entry start table; compare disputed calendar conventions explicitly.
At a glance
- Conversion field
- Leap-month flag
- Lookup starts
- 12 month entries
- Known policy edge
- 2033–2034 ambiguity window
A leap month is not February 29
A Gregorian leap day adjusts the solar calendar by one day. A lunisolar leap month inserts an additional month.
Confusing the two changes the question before any mansion lookup begins.
Identify the repeated month
Keep the converted month number and the leap marker separately. The month marker distinguishes the additional month from the ordinary one.
The product’s mansion lookup uses the numbered month’s starting entry and lunar day.
Do not apply an unstated split rule
Some explanations discuss special treatment within a leap month. Such a rule must be named and sourced before use.
It must not be silently added to OpenFate policy 1.0.0, whose engine uses the converted month and day directly.
Why 2033 needs a policy
The current engine labels an ambiguity window from 2033-08-25 to 2034-03-19 and identifies its winter-solstice-priority convention.
That metadata explains a deliberate choice; it does not establish that every calendar source must return the same answer.
Compare a disputed result
First compare the leap flag and month number, then the day and month-start mansion.
If the calendars differ, keep both conventions visible. Do not use a personality description to decide which astronomical calendar calculation is correct.
Worked comparison format
Label two records “ordinary month 8, day 5” and “leap month 8, day 5” rather than writing only “8/5.” Apply the declared policy to each record and preserve the leap marker beside the output.
Identical month and day numerals do not prove that the two civil dates or calendar states are interchangeable.
Sources and editorial basis
- 宿曜經 · T1299 卷上卷上「大唐月建圖」: twelve numbered month columns; no claim that it validates modern 2033 policy.Historical text for mansion names and calendar tables. Its table is not identical to OpenFate’s modern 27-mansion lookup; it does not establish predictive validity.
- OpenFate Sukuyo calculator and calculation conventionEditorial code check 2026-09-10, policy 1.0.0: leap-month; policy.ts, constants.ts, engine.ts and traditional-cycles.ts. Product settings are not an independent historical authority; practical examples are OpenFate editorial applications.Product-specific input and output reference. Editorial checks also inspected policy 1.0.0 and the month-start constants on 2026-09-10; those implementation checks are not an independent traditional authority.
- NAOJ: 「旧暦」ってなに？NAOJ question 3-4: old-calendar definition, month starts, leap months and no current official Japanese lunisolar calculation.Calendar-method reference: modern Japanese old-calendar calculations are not one official standard. This supports the calendar distinction, not divination.