Though most collectors take dates on coins for granted, dates on coins did not come into wide-spread use in Europe until the 16th century.
Today, although most oriental coins are dated, they are not always dated with an AD date.
Both Japan and Taiwan date their coins by the number of years the emperor or government has been in power.
Note that the gannen (first year) is generally not a full year long.
For example, Showa 1 started December 26, 1926, and Showa 2 started a week later (January 1, 1927).
June 20th, 2015 would be , while in the US it would be .
ten commandments of dating summary - Japan coin dating calendar
For the case of Japanese, they use two calendars: the modern Gregorian calendar and the Japanese traditional calendar, called the nengo (年号) system.
These two Hansatsu notes were issued by the Hyogo Amagasaki-han clan in about 1777.
The clan controlled the region around Amagasaski castle in what was then Settsu Prefecture, which is located near Osaka. Their castle was located near where two rivers flowed into the ocean and was connected to the river by a moat.
The Japanese calendar years increment by 1 every January 1st (the same as the Gregorian calendar), not on the anniversary of the Emperor's reign.
Currently in Japan, both the Gregorian calendar years (e.g., 2004) and Japanese nengo are used.