A Reiwa year is the Western year less 2018, so 2026 is Reiwa 8, and Heisei, Shōwa, Taishō and Meiji work the same way with 1988, 1925, 1911 and 1867. The month and day matter only around four changeover days, in 1912, 1926, 1989 and 2019. On 2 October 2026, by our search of e-Gov, Japan's official law database, 302 laws and regulations in force still named a Heisei year the era never reached, the latest being Heisei 72, 31 March 2060.
Before 1873 no offset works at all, because Japan used a lunisolar calendar until the last day of 1872.
Each era's first day rests on its own instrument
No single rule says when an era begins. The National Archives of Japan holds the signed originals of the edicts for 1912 and 1926, catalogued as 「明治四十五年七月三十日以後ヲ改メテ大正元年ト為ス」 and 「大正十五年十二月二十五日以後ヲ改メテ昭和元年ト為ス」. Each emperor died on the day named, and the new era ran from that day, so 30 July 1912 was Taishō 1 and 25 December 1926 was Shōwa 1.
Heisei came by cabinet order. The order of 7 January 1989, the day the Shōwa emperor died, reads 「元号を平成に改める。」 and took effect 「公布の日の翌日」, the day after promulgation. Shōwa 64 therefore lasted seven days, and Heisei began on 8 January.
Reiwa's order took effect on the day after the abdication law's effective day, 30 April 2019. That law's Article 2 says the emperor abdicates on that day and 「皇嗣が、直ちに即位する」, the heir accedes immediately, so 1 May 2019 was both the new emperor's first day and Reiwa 1, written 令和元年. The prime minister's statement on 1 April 2019 gave the name's source as a line in the Man'yōshū's preface to its plum-blossom poems, and at his press conference he said it was 「歴史上初めて国書を典拠とする元号」, the first era name in history drawn from a Japanese classic.
The Era Name Act's entire text is two sentences
Since 1979 every change has followed the Era Name Act (元号法), promulgated and in force on 12 June 1979. Its main text reads in full: 「元号は、政令で定める。」 and 「元号は、皇位の継承があつた場合に限り改める。」 An era name is set by cabinet order, and changed only when the throne passes. A supplementary provision states that Shōwa counts as having been set under the first of those sentences.
The changes before 1979 were made by imperial edicts. The rule of one era per reign dates from 1868, when, the National Archives says, the Meiji edict renamed Keiō 4 as Meiji 1 and set 「一世一元」 at the same time.
Meiji's starting day has two defensible dates
The Meiji edict was issued on the 8th day of the 9th lunisolar month of Keiō 4, which was 23 October 1868. It made the whole of that year Meiji 1, back to its lunar new year, 25 January 1868. Converters split on which date to use. ICU, the international calendar library behind the Japanese calendar in many browsers, starts Meiji on 23 October. The National Astronomical Observatory of Japan's calendar database labels the 9th month of 1868 as Keiō 4 and the 10th as Meiji 1. Our Japanese era converter follows the edict's wording and starts Meiji on 25 January. For any date before 1873 the choice matters less than it looks, because no arithmetic converts those dates anyway.
A lunisolar date's Gregorian reading lands weeks early
Japan switched to the solar calendar by an edict issued on 9 December 1872, 23 days before the change: the lunisolar Meiji 5, 12th month, 3rd day became 1 January 1873. The National Archives' account of it records the decree, 太政官布告第337号. Meiji 5's 12th month had only two days.
We took the observatory's month-start dates for 1868 to 1872 and read each of the 1,803 days from 25 January 1868 to 31 December 1872 two ways: by its real lunisolar label, and by taking the same numbers as a Gregorian date. On 1,795 of those days the Gregorian reading lands between 22 and 53 days early, 38 days at the median. The other eight labels are not real Gregorian dates at all, and 58 of the days fall in leap months, which have no Gregorian counterpart.
ICU prints a Japanese era label for every one of those days, but with the Gregorian month and day, and on none of the 1,803 does its label match the date people were actually writing. For 10 April 1870 it prints 明治3年4月10日. That day was really 明治3年3月10日, and the real 明治3年4月10日 was 10 May 1870.
Our converter's weak spot was the statute's own numerals
Our converter and ICU agreed on the era, year, month and day of every one of the 57,708 days from 1873 to 2030, before its fixes and after. What it could not do, until 2 October 2026, was read dates the way Japanese law writes them, in kanji numerals. We took 50 full date phrases from e-Gov texts and it read none of them as written; with the numerals changed to digits it read all 50. It also could not read back its own first output row, the full kanji date with the weekday, on any day, and its page said the eras before 1979 began on the day of accession and the two since on the day after, which is wrong for Meiji and for Reiwa. It now reads kanji numerals, a weekday in brackets, a year and month without a day, and the pension service's era codes.
Unicode's ㋿ sits apart from the older era squares
The same date can be written several ways. 令和元年 uses 元年 for the first year. Statutes write numerals in kanji, as in 平成三十一年四月三十日. Everyday forms use the letters M, T, S, H and R. Fonts carry single-character squares for the era names: ㍻, ㍼, ㍽ and ㍾ for Heisei, Shōwa, Taishō and Meiji at U+337B to U+337E, and ㋿ for Reiwa at U+32FF. Unicode 12.1, released on 7 May 2019, added that one character and nothing else. The code point after Meiji's square, U+337F, was already taken by ㍿, the square for 株式会社.
Japan Pension Service forms use numbers instead: the birth-date field on one of its notification forms offers 5 for Shōwa, 7 for Heisei and 9 for Reiwa. In a 2024 written answer to the House of Representatives, the Cabinet said it was aware that the pension service's form-filling program offered no Western year in its date menus, and that it did not consider that public documents must always carry a Western year as well as the era, because public bodies have by custom used the era as the rule.
Japan's statute book carries Heisei years the era never reached
Anything written before May 2019 that looked further ahead than April could name a date such as 平成32年, because nobody knew when Heisei would end. For government use the question was settled on 1 April 2019, when the ministries' liaison conference on the change agreed that a Heisei year shown for a date after the change 「有効なものであり」, remains valid, and that applications to ministries written that way would be accepted. The agreement covers ministry documents and filings to ministries. For a private contract or card, all that can be said is that, counted on, Heisei 32 means 2020.
The agreement also said laws would not be amended for the era alone, and that the wording would be corrected when a law was next amended for another reason. We searched e-Gov's current texts for every Heisei year from 32 to 130 on 2 October 2026. The phrase 平成三十二年 alone appeared in 218 texts. Across all the years, 325 texts named a Heisei year the era never reached, and 302 of them were in force, 114 of those Acts of the Diet. Many are spent transitional provisions rather than live deadlines.
Some are not spent. The Act for the agriculture, forestry and fisheries growth fund has been amended three times in the Reiwa era and still sets a date of 平成四十五年三月三十一日, 31 March 2033. The 2019 law for the Osaka Expo was promulgated four days before Heisei ended with 平成三十七年 in its title and was renamed for 令和七年 in 2021, but the ministry ordinance that implements it, issued after Reiwa began, still carries the Heisei title. The furthest date is in a cabinet order on the Kansai and Osaka airports, which requires a lease to run until at least 平成七十二年三月三十一日, 31 March 2060.
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