Need Help Determining Date On A Japanese 100 Yen Coin

macmercury, I'm a little confused by your answer. Are you saying that my coin's date is 1989?

In the Japanese coin series, for the first year of a new emperor, they do not write "year 1". They write "year first". The character for "first" is called gen and looks kind-of like a mathematical pi with a line on the top of it. I'm not quite sure why they do this; I can only assume "cultural reasons" is the answer.

Thus, the coin is from the First year of the Heisei emperor, or 1989.

Because the standard Western-numeral shorthand for "first" cannot be written without using Latin letters and because the Latin letters used for doing this varies from language to language (e.g. it is "1st" in English, but "1er" in French and "1.o" in Spanish), they choose to use the Japanese character instead. 100 yen coins from Heisei Year 2 onwards revert to using Western numerals for the date. example. We see the same character on Reiwa 100 yen from Year 1 (2019).