Dating Japanese maps

Here's one for the map librarians out there. (And probably for catalogers too!)

Japanese maps typically have dates on them that use the reign of the sitting emperor for the year. So 昭和25年 would be the 25th year of Showa, aka Hirohito. Since he took office in 1926 (year 1), a map showing the above date would be from 1950.

This all comes up because it turns out I can read numbers and dates in Japanese now too. Pretty cool eh?