Hōryaku calendar (Hōryaku-reki) — Japanese lunisolar calendar published 1755

Characteristics

Like other Japanese pre‑modern calendars, the Hōryaku calendar organized time by moon phases while inserting occasional intercalary months to keep months aligned with the seasons. Its main features include:

• Moon‑based months: each month begins with a new moon, so month lengths are typically 29 or 30 days.
• Intercalation: extra (leap) months are inserted periodically so the lunar months stay roughly synchronized with the solar year.
• Astronomical calculations: dates were determined by contemporary observations and formulae adapted from East Asian calendrical traditions.

History and usage

Published during the mid‑18th century, the Hōryaku calendar was one among several successive calendar reforms in pre‑modern Japan. Calendars of this period governed civic and ritual life: they were used to set festival dates, regulate agricultural activities and mark official ceremonies. For broader context on calendars of the region see Japanese calendar systems and general principles of the lunisolar calendar.

Because the Hōryaku calendar is tied to lunar phases and periodic corrections, its month names and start dates do not coincide exactly with the modern Gregorian months. Historians and archivists frequently convert Hōryaku dates to Gregorian equivalents when working with 18th‑century Japanese records.

Notable facts and distinctions

The Hōryaku calendar is notable mainly as a mid‑18th century Japanese reform and as part of the wider genka‑style calendrical tradition recorded in Japanese sources. It remained one of the local calendrical methods used until the adoption of the Western (Gregorian) calendar in the 19th century, which eventually standardized civil dating in Japan.