Sukuyo study guide

Sukuyo lunar calendar (kyūreki): conversion rules and checks

A Sukuyo lunar calendar converts a civil date under a declared Japanese lunisolar method before lookup; kyūreki is not one universal algorithm.

Editorially reviewed: Sep 10, 2026 · Reviewed by OpenFate · Editorial Methodology

Overview

The new-moon civil date and month-numbering rule are part of the calculation, not optional details beneath a generic old-calendar label.

A Gregorian birthday does not directly supply the lunar month and day used in the mansion table.

The conversion must be completed before counting from a month’s starting mansion.

NAOJ explains that several lunisolar calendar methods existed and that there is no current official Japanese lunisolar-calendar calculation.

A modern website should therefore disclose its method rather than imply that the word kyūreki settles every edge case.

Policy 1.0.0 starts lunar day one on the Japanese civil date containing the astronomical new moon.

Principal solar terms help number months, with a winter-solstice-priority rule for a 13-month span. These are product settings.

An astronomical event near midnight can fall on different civil dates in different timezones.

A Chinese-calendar conversion should not be substituted for a Japanese convention merely because both display lunar month and day.

Keep the Gregorian date, lunar year/month/day, leap flag and convention version. Compare these before the mansion name.

For disputed cases, retain the uncertainty and consult the calendar-method source rather than modifying the date until the outputs match.

When a new moon occurs near Japanese midnight, compare the event instant, its Asia/Tokyo civil date and the month assignment used by each calendar.

A one-day difference can propagate into lunar day and mansion; it should be explained at the calendar step, not repaired in the interpretation.

宿曜經 · T1299 卷上
卷上「大唐月建圖」: table requires calendar month/day; modern conversion convention is a separate product rule.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 convention
Editorial code check 2026-09-10, policy 1.0.0: kyureki; 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.

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