Many Japanese releases include a code representing the item's release date in year-month-day format. Sometimes these appear in a box on the back of the sleeve, while other times they will appear on the obi (the information sheet wrapped around the outside of the packaging), and there is often more than one date listed. This is an overview of how to interpret these date codes.
Thanks to discogs.com user jasmithers for much of this information.
Sometimes a release code will use a letter in the place of the year, such as Ｈ • ９ • ５. I'm aware of the following letter-to-date translations:
Ｎ=1984; Ｉ=1985; Ｈ=1986; Ｏ=1987; Ｒ=1988; Ｅ=1989; Ｃ=1990.
Using this list we can see that our example code translates to September 5, 1986.
Many Japanese releases have two dates encoded on the packaging. These dates are a part of Japan's fifty-year-old saihan seido system, which allows publishers to set a minimum retail price (typically printed on the packaging) for which an item may be sold.
When two dates appear, the earlier date is the item's release date and the later is the date after which the saihan pricing restrictions are lifted. A tpyical example is Ｘ ～ ８ ８ • ２ • ２ ４ Ｏ • ２ • ２ ５, which hides the earlier date (February 5, 1987) behind a letter code.
The date pairs do not always appear together. On one release of Propaganda's A Secret Wish the saihan date 再＇９ ６ • ５ • ９ まで is on the front of the obi, while the earlier release date ９ ４ • ５ • １ ０ （ ９ ４ • ５ ） Ⓧ Ⓨ is on the back. (In this example, 再...まで translates roughly to "resale...until".)