The maximum year-of-era should be 1868, not based on the start of the current era. This corresponds to the end of Seireki.

This issue covers all work to confirm the boundaries between eras, and whether these are part of the spec of JapaneseEra or only defined in the config file (which I assume can be replaced by users to enter a new era)

JSR 310 issue: https://github.com/ThreeTen/threeten/issues/325