Japanese Calendar
Outline
Japanese Calendar has been used in Japan. Year is expressed with era. Chinese and Arabic numerical are used for year, month and day numbers, and zodiac signs are also used for year and day. Japanese Calendar is a lunisolar calendar before 明治5年12月2日 (A.D. 1872-12-31 in Gregorian Calendar / JD = 2405158.5), and is a solar calendar (month and day number in Gregorian Calendar are used) after 明治6年1月1日 (A.D. 1873-01-01 in Gregorian Calendar / JD = 2405159.5).Both eras of the southern court (including Daikakuji-tō 大覚寺統) and of the northern court (including Jimyōin-tō 持明院統) are supported.
Temporal Range
From: 神武天皇元年1月1日 (B.C. 0660-02-18 in Julian Calendar / JD = 1480406.5)
To: 平成94年12月31日 (A.D. 2082-12-31 in Gregorian Calendar / JD = 2481859.5)
Source
Days
Place of leap month and number of days in a month in Lunisolar calendar (before 明治5年12月2日) and number of days in a month in solar calendar (after 明治6年1月1日) is according to following sources.
- From 神武天皇元年1月1日 To 允恭天皇33年12月29日
内田正男 (Uchida, Masao). 日本書紀暦日原典 新装版 (Source book of days in Nihon Shoki, new edition). 雄山閣 (Yuzankaku). - From 允恭天皇34年1月1日 To 明治5年12月2日
内田正男 (Uchida, Masao). 日本暦日原典 第四版 (Source book of days in Japan, 4th edition). 雄山閣 (Yuzankaku). - From 明治6年1月1日 To 平成94年12月31日
National Laws in Japan; 明治五年太政官布告第三百三十七号（改暦ノ布告）(Daijō-kan decree No. 337 in Meiji 5 (decree about calendar reform)) and 明治三十一年勅令第九十号（閏年ニ関スル件）(Edict No. 90 in Meiji 31 (Edict about leap year))
Eras
From "神武天皇" To "文武天皇"
Era names and started dates are according to 日本書紀 (Nihon Shoki) and 続日本紀 (Syoku Nihon-gi) (Both sources are contained in 日本国史大系 (Supplementary compendium of Japanese history) published by 吉川弘文館 (Yoshikawa Koubunkan)).
(1) When name and started date of a era are described in the sources, the era is according to the description (大化, 白雉 and 朱鳥).
(2) For eras which do not have name, emperor's (or Queen's) names are used instead of era name. In these case, it is assumed that new era started from 1st day of 1st month in the 1st year of the era (not enthronement date of the emperor). However, since 11th year of 持統天皇 (year zodiac signs is 丁酉) descripted in 日本書紀 and 1st year of 文武天皇 (year zodiac signs is also 丁酉) in descripted in 続日本紀 are the same year, it is assumed that 文武天皇 era started from enthronement date of the emperor 文武. 弘文天皇 era is supported only for input data (these date is expressed as 天武天皇 era for output).
After "大宝"
Era names and started dates are according to 国史大辞典 (Great Dictionary of National History) published by 吉川弘文館 (Yoshikawa Kobunkan). However, when stated or terminated date of era are unclear, these dates are assumed as follows. Please refer to Note for influence of these assumption on calendar conversion.
(1) When descriptions in the dictionary are different between eras before and after the era change, the more clear one is adopted.
from 観応 to 文和: Although there is description that "文和 era started from 正平 7-9-27 because 観応 era was disused and return to previous era 正平 in 正平 6-11 in explanation about 文和, there is description that "観応 era is terminated on 観応 3-9-27 and 文和 era started" in explanation about 観応. Since the date that 観応 disused is unclear, it is assumed that 観応 is directly changed to 文和 on 観応 3-9-27.
from 天授 to 弘和: Although there is description that "弘和 started form any date between 天授6-7 and 天授6-6" in explanation about 弘和, there is description that "弘和 started on 天授7-2-10" in explanation about 天授. the more clear description (弘和 started on 天授7-2-10) is adopted.
from 元徳 to 正慶: Although there is description that "正慶 started form 元弘 2-4-28" in explanation about 正慶, there is description that "元徳 was continued to use until 元徳 4-4-28 in the northern court". It is assumed that 正慶 started from 元徳 4-4-28, when the northern court of Japanese Calendar is specified for the conversion.
from 元中 to 明徳: Although there is description that "元中 is terminated and 明徳 started in 9-閏10 ("閏" means leap) because of unifying both courts", the detail date is not descripted. Is is assumed that 明徳 started from 元中 9-閏10-5, according to description that "unifying both courts was completed on 元中 9-leap10-5" in explanation about unifying the Southern and the Northern Courts.
(2) When stared date of a era is not fixed, date introduced in the dictionary as the earliest date adopting the era in an exist document is used as the started date.
from 正平 to 建徳: It is assumed that 建徳 started on 正平 25-2-5 which is introduced in the dictionary as the earliest date adopting 建徳, and appeared in a document 大徳寺文書 (Daitokuji-Monjo).
from 建徳 to 文中: It is assumed that 文中 started on 建徳 3-4-28 which is introduced in the dictionary as the earliest date adopting 文中, and appeared in a document 金剛寺文書 (Kongouji-Monjo).
from 弘和 to 元中: It is assumed that 元中 started on 弘和 4-11-5 which is introduced in the dictionary as the earliest date adopting 元中, and appeared in a document 栄山寺文書 (Eizannji-Monjo).
Format
Additional specifiers relating with Chinese numerical and zodiac signs can be used. Please refer to Format of Japanese Calendar.
Note
Specification of era
For Input
Eras of both the Southern and the Northern Courts are acceptable. When 紀元 or 皇紀 appears as era name in the input text, expression of the year is interpreted as Japanese imperial year.
For Output
Please specify Eras of the Southern or the Northern Courts for output expression. Japanese imperial year is shown using specifier "yJ" (please refer to Format of Japanese Calendar).
Calculation of era and year
For Input
dates before era was changed: both the previous and the new era are acceptable if the date is in the year that the era was changed.
e.g. Although 昭和 started from 大正 15-12-25, expression of 昭和 1-1-1 (actually 大正 15-1-1) is acceptable.
dates after era was changed: the previous era is acceptable. Year with the previous era is calculated assuming that the era is continued.
e.g. Although 平成 started form 昭和 64-1-8, expression of 昭和 80-1-1 (actually 平成 17-1-1) is acceptable.
For Output
Era and year are expressed according to date that era was changed, regardless of input text
Examples
元中 started from 弘和 4-11-5.
Both expressions of 弘和 4-11-4 and 元中 1-11-4 are acceptable as the date that the day before era was changed, and converted to expression of A.D. 1384-12-16 in Julian Calendar. While A.D. 1384-12-16 in Julian Calendar is converted to expression of 弘和 4-11-4 in Japanese Calendar (Southern Courts).
In the same way, both expressions of 弘和 4-11-5 and 元中 1-11-5 are acceptable as the date that era was changed, and converted to expression of A.D. 1384-12-17 in Julian Calendar. While A.D. 1384-12-17 in Julian Calendar is converted to expression of 元中 1-11-5 in Japanese Calendar (Southern Courts).
