This is the dating system which is most commonly encountered on the nakago of Japanese swords. In modern times as each emperor takes the throne, his reign is given a name which is represented by two kanji. In older times an emperor might give names to several eras or reign periods within his reign. Also he may continue to use a reign name given by his forebear for some time after his accession to the throne. Reign periods or eras therefore, do not necessarily correspond directly with the reign of a particular emperor.
The year-date is simply measured as a year count from the beginning of the relevant neng˘ or reign period.
As an example the EISHď reign period began in the year 1504 CE.
It is represented by the kanji
If on a blade the date is shown as EISHď 12 NEN then this simply means (assuming it is not gimei!) that it was made in the twelfth year of the period EISHď.
Note that as reign periods have no year zero, the correct way to calculate the western equivalent date in this case is to take the beginning year of the reign (1504), add the 12 (=1516), then subtract 1 (=1515). So EISHď 12 = 1515 CE.
An exception to this numbering system is the first year of each era. The first year of any reign period is called GANNEN so the first year of EISHď would be represented as
Also note that between c.1332 and c.1392 CE there were two courts (Northern and Southern) ruling Japan, so for any given year in this period there are two separate neng˘.
A full Neng˘ List covering TENKI (1053 CE) to HEISEI (present day) is available.
Occasionally following the neng˘ and the year number, is another number followed by GATSU and HI . This is the number of the month and the kanji for 'day'.
A full date inscription on a sword nakago might read as follows:
Tenp˘ jűsan nen ni gatsu hi
This translates as "A day in the second month of the thirteenth year of Tenp˘"
You can see from the Neng˘ List that the Tenp˘ reign period began in 1830 CE, so adding 13 and subtracting 1, the western equivalent date for this blade would be 1842.
Instead of a month there may be one of the kanji indicating the season, together with HI . This would translate as " A day in spring/summer/....."
For practice in reading Japanese Neng˘ dates see the Practice Date Inscriptions Page
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