How Sukuyo calculation works: birth dates and the 27-mansion table

Start with the input calendar

Use the date format requested by the calculator. A Gregorian date and a lunar month/day with the same numbers do not identify the same day. Record the original input rather than trying several birthdays until a preferred mansion appears.

For this product convention, birth time is not an input to the mansion assignment. Do not invent a noon birth time or adjust an overseas birthday to Tokyo on your own: follow the date-entry instructions and distinguish a date-only record from an exact birth instant.

Convert the date before choosing a mansion

OpenFate policy 1.0.0 uses the Japanese civil date containing an astronomical new moon as lunar day 1. Principal solar terms at 30-degree intervals help assign months; a winter-solstice-priority rule determines leap months in a 13-month span.

A generic lunar calendar can use a different civil timezone or leap-month convention. Keep its month, day and leap-month flag in the comparison. Do not replace a calendar conversion with a Western zodiac date range.

Apply the month-start table

For lunar months 1 through 12, OpenFate’s starting mansions are 室, 奎, 胃, 畢, 参, 鬼, 張, 角, 氐, 心, 斗 and 虚. Count lunar day 1 as that starting position, then move forward one position for each subsequent day in the 27-mansion sequence.

A worked table example, not a Gregorian birthday conversion: if the converted lunar date is month 1, day 1, the result is 室. Day 2 advances to 壁; day 6 wraps around to 昴. The example verifies the lookup only. It does not establish which civil date produced that lunar date.

Why do two Sukuyo calculators disagree?

Compare the earliest step at which they diverge. If the lunar dates differ, inspect the civil-day, new-moon and leap-month settings. If the lunar dates agree but mansions differ, compare the month-start table and whether the sequence has 27 or 28 entries.

There is a concrete source difference to watch: the T1299 volume 1 大唐月建圖 begins its first-day row with 虛, whereas OpenFate’s modern month-start table begins with 室. The product table is therefore not presented as a verbatim transcription of that historical table.

• Compare original dates and calendar labels.
• Compare converted lunar month, day and leap flag.
• Compare month-start mansion, sequence and day-counting rule.
• Keep unresolved differences visible instead of declaring one result universally correct.

What this calculation does not supply

A date-based mansion label does not supply the Moon’s exact birth longitude, an individual quarter of a mansion or a scientific personality score. Those are not hidden extra outputs of the table.

Use the linked calculator for a real date and retain its displayed settings. Once the calculation is settled, interpretive material can be read separately. A repeatable label does not make a prediction empirically proven.