https://reference.discogs.com/wiki/japanese-release-dates#date-format

Early Japanese CDs did not write out the year number for release dates, but used a Latin letter to represent the year. H, for example was 1986 while N was 1984. 

This standard fell out of favor in the early 90s, but it is a common cause of confusion when buying old Japanese CDs. The letter-year relationships are listed in the link above; it would clear up some confusion or speed up data entry if, for example, entering H into the year field on a release automatically corrected to 1986.