Japanese Era (Nengō) to Year Converter — Date Your Sword

A nengō (年号), or gengō, is a Japanese era name proclaimed by the imperial court. Before 1868 eras changed frequently — at an emperor’s accession, after disasters, or at auspicious calendar points — so most eras last only a few years. Since Meiji (1868) each era matches one emperor’s reign. Dates on Japanese swords, documents and certificates are all written in nengō.

Take the first Gregorian year of the era, add the era-year number, and subtract one. Example: Tenshō began in 1573, so Tenshō 10 = 1573 + 10 − 1 = 1582. The converter above does this automatically and flags the sword period.

During the Nanbokuchō period (1336–1392) Japan had two rival imperial courts, each proclaiming its own era names. A sword dated Shōhei 22 (Southern Court) and one dated Jōji 6 (Northern Court) were both made in 1367. Swordsmiths generally used the era of whichever court controlled their province.

They are the standard classification of Japanese swords by production era: Kotō (“old swords”, before 1596), Shintō (“new swords”, 1596–1781), Shinshintō (“new-new swords”, 1781–1876), Gendaitō (1876–1945) and Shinsakutō (contemporary work). The boundary years follow the Keichō era (1596) and the An’ei/Tenmei transition (1781).

Generally yes. A legible date alongside the smith’s signature makes attribution and NBTHK certification easier and confirms the blade’s place in the smith’s career. Undated blades are common, however — many smiths only dated commissioned works. Browse our certified antique swords, most with documented dates and NBTHK papers.