Let's say you have a data frame with Japanese Calendar Years with Nengo like this.
If you want to convert them into Common Era, you can make use of mutate with case_when function like this.
nengo == "昭和" ~ year + 1925,
nengo == "平成" ~ year + 1988,
TRUE ~ 0)
It would look like this on the mutate dialog.
Here is the resulting data frame with a new column with the years in Common Era.