Occasionally I read texts on Japanese history and always struggle to figure out whether the dates are in the old or new system, specifically whether something like 四月 corresponds to April as in the western calendar or to the fourth months in the old calendar. In my experience both options are used more or less randomly.

Is there any way to make sure from the language or are there any other indications (publication year, type of book,....) which of the two dates is meant?

  • 1
    Is an old system meaning "lunar/Chinese calendar?" or "Imperial?"
    – user20428
    Mar 30, 2017 at 7:10
  • 1
    @SadaharuWakisaka Anything that is not like the current/western system. Mar 30, 2017 at 11:28
  • Now i got your question.
    – user20428
    Mar 31, 2017 at 0:04

2 Answers 2


I'm not aware of any ironclad rule for this. You can generally assume the old calendar is used as long as there is a 元号 and it's a pre-幕末 event. For example Wikipedia's guideline says:


大政奉還 happened on 1867年11月9日 in the Gregorian calendar and 慶応3年10月14日 in the old calendar. Most articles about 大政奉還 listed here either silently use the date in the old calendar (i.e. 10月14日) or explicitly lists the two dates in both systems.

徳川家重 was born on 1712年1月28日 in the Gregorian calendar and 正徳元年12月21日 in the old calendar. However 正徳元年 mostly overlaps AD 1711 in the Gregorian calendar. Then was he born in 1711 or 1712 in 西暦? The result is the strange discrepancy about his birth year found in various sources. One source says he was born in 1712*, where this asterisk is described in the book's 凡例 page as:


But I don't think this symbol is widely used elsewhere.

Another source says he was born in 1711, and says in its 凡例 page:

  1. 西暦と和暦の関係は、厳密に月日まで比定せず、原則として改元年号で示した。
  2. 明治5年(1872)の改暦以前の月日は原則として和暦によった。西暦と和暦には月日において若干のずれがあり、とくに和暦12月は西暦換算で翌年になる場合が多いが、本書では和暦によった。

One source even says he was born on 1711年12月21日, which seems highly confusing (or wrong) to me...

So you'll have to be very cautious when the year is expressed in four digits. Basically you'll have to figure out the rule of each publication from the preface or such.

  • A digression, but is the old Japanese calendar exactly the same as the traditional Chinese calendar, or are there some subtle differences?
    – xuq01
    Jun 2, 2017 at 8:13

Japanese people don't care much about it. For example 沖田総司 Souji Okita a member of 新選組 Shinsen-gumi died in 慶応4年5月30日 = July 19th 1868, this is probably your question.

Japanese calendar is confusing. Before the internet, most people forced the imperial calendar. Now many people found inconvenience and shifting into western calendar.

Imperial calendar(元号): The new period starts with the new emperor. 平成 1989 - 2018, the current emperor told us his abdication on 2019. It is going to be a new period.

Any documents from Public office are written in this way.e.g Driving License expire 平成31 and I have no crew because I don't remember year in 元号. My grandmother was born in 大正15年 hard to count her age because of sum of 平成, 昭和 and 大正, it is very simple to calculate if she could tell us born in 1926. Most commercial companies no longer use Imperial calendar, Insurance companies, Bank and Stock companies. They were fed up with the problems became real after the shift of 平成 new imperial year such as no 昭和105年 and etc. News sometimes tells us "The new bullet train, the linear motor car" plans to start commercial drive in year of 30!" (sorry it's strange English but no other way to tell) So people heard this it comes in a years or 13 years. This is annoying.

Lunar calendar 旧暦: it's different from Chinese calendar and roughly a month or two late from western calendar, as you know it's based on moon face, the full moon day is 15日, the new moon is 1日, month ends 29 or 30. This had been s an official until 明治5年12月2日 = December 31th 1872. The installation of Gregorian calendar in Japan was 1873 January 1st. So Meiji 5 has 26 days shorter than other years. Before this happens every documents in Japan were written in Imperial year and Japanese lunar calendar.

Japanese lunar calendar is somewhat fuzzy. It's very different from Chinese. In some suburban places, they has been hold ひな祭り(hina-matsuri) March 3th, it was on the lunar calendar, so they found the way it's roughly a month late then it'd be held on April 3th.

Before the Meiji period, it would be the lunar calendar with imperial year which is very hard to know the accurate date in western calendar.


For year 2017 = 平成/Heisei 29 = 昭和/Showa 92 = 大正/Taisho 106 = 明治/Meiji 150

For Month in modren/old Japanese alias

January 1月:睦月(むつき)

February 2月:如月(きさらぎ)

March 3月:弥生(やよい)

April 4月:卯月(うづき)

May 5月:皐月(さつき)

June 6月:水無月(みなづき)

July 7月:文月(ふみづき)

August 8月:葉月(はづき)

September 9月:長月(ながつき)

October 10月:神無月(かんなづき)

November 11月:霜月(しもつき)

December 12月:師走(しわす)

  • 1
    Handling old japanese dates in computers is difficult so I had written a new Java-based library (Time4J) in order to make interpreting of such dates in pre-meiji-era easy. Example for the death date of Souji Okita see this programming code Aug 21, 2017 at 12:31
  • Sorry but I am not good at coding. And I wish you not to take the date of very old events, because I am pretty sure that they had mistaken to describe the date in their system. So no points to be serious, but I know something else. Dutch VOC had kept quite accurate records of Japanese history after 1600. Their dairy are worth to read. I have one book Dutch publisher described about how they were treated and how the japanese people at that time. It is amazingly clear and convincing.
    – user20428
    Aug 22, 2017 at 14:04
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    I have based my work on the chronological tables of Paul Y. Tsuchihashi published in year 1953. And also compared with the work of a Japanologist from Germany (Rainer Zöllner). Both have done their best to record all Japanese dates back to midage. Aug 23, 2017 at 14:06

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