Historical Japanese calendars
Calendar history
Japan used a succession of lunisolar calendars before adopting the Gregorian calendar in 1873. Calendar knowledge reached Japan from China through the Korean peninsula. The Nihon Shoki chronicle records under the year 553 an attempt to recruit a calendar specialist from Baekje and obtain calendar books.
In a lunisolar calendar, months followed the Moon's cycle while leap months kept the numbered year in step with the seasons. Japanese calendars distinguished 30-day “large” months from 29-day “small” months. Their sequence changed from year to year. A leap month, uruuzuki, added another lunar month. Thus “month 4” in an old document is not automatically Gregorian April.
That changing sequence mattered in everyday life. Daisho-reki, calendars showing large and small months, could hide the relevant numbers in pictures or wordplay. They combined a practical reminder of month lengths with visual entertainment. Month lengths were also important for arranging payments and other month-end business.
Selected calendar systems and their first years of use
| Calendar | First year used in Japan | Place in the sequence |
|---|---|---|
| Senmyo | 862 | Chinese-derived system used for more than eight centuries |
| Jokyo | 1685 | Reform associated with Shibukawa Harumi |
| Horeki | 1755 | Later Edo-period reform |
| Kansei | 1798 | Later Edo-period reform |
| Tenpo | 1844 | Last official lunisolar system before Gregorian adoption |
| Gregorian | 1873 | New civil calendar introduced in the Meiji period |
The table gives years of use, which can differ from the year a reform was approved. The Jokyo reform, for example, was authorised in 1684 and used for 1685. Harumi adapted the Chinese Shoushi calendar through observation and adjustments that included the longitude difference between China and Japan. Later reforms incorporated Western astronomy.
A calendar was also a published object containing more than dates. Guchureki, detailed annotated calendars, carried seasonal information and judgements about days. Their owners sometimes used the available spaces for diary entries. Simplified calendars written in kana, the Japanese syllabic script, and the spread of printing brought calendrical information to wider readerships. A surviving calendar can therefore preserve both the year's published information and a reader's record of daily life.
The 1873 change
The Meiji reform cut short the old calendar's twelfth month. Its second day was the last day observed under the former civil system. The following day, which would have been Meiji 5, month 12, day 3, became January 1, 1873, or Meiji 6, month 1, day 1. The day sequence continued while the calendar labels changed.
| Position at the reform | Japanese dating | Gregorian equivalent |
|---|---|---|
| Last day under the old civil calendar | Meiji 5, month 12, day 2 | December 31, 1872 |
| First day under the new civil calendar | Meiji 6, month 1, day 1 | January 1, 1873 |
The reform was enacted through Dajokan Proclamation No. 337 of Meiji 5. It replaced the civil calendar without abolishing era-year dating. A Meiji date must therefore be read with its calendar context: the same style of year designation appears on both sides of the change.
Old-calendar information continued beside the new dates in official calendars through the calendar for 1909. Gregorian adoption was a sharp administrative change, but lunar dating and seasonal terminology remained familiar to readers.
Historical lookup
A historical conversion begins with the date as written: era name, era year, month, day and any leap-month designation. Keep that transcription beside the converted result. The ordinary month and a leap month bearing the same number are separate months, so dropping the leap marker can move the answer by an entire lunation.
The Japanese Luni-Solar Month Database provides a historical lookup with a leap-month field and a choice of civil-calendar output. Use the era and month information from the record to locate the corresponding month, rather than extending a modern Chinese calendar backwards.
By default, the database reports Gregorian dates from October 15, 1582, and Julian dates before that boundary. Select Gregorian output explicitly when Gregorian equivalents are required throughout a project. Some historical era boundaries are uncertain, so an entry close to a change of era may also need the original document's political and geographical context.