regnal

In Japan, nobody uses the emperor's birth name; they just use the regnal name.

This meant that in discussing conflicts between kingdoms, the date would have to be given in the regnal years of all the kings involved.

The other approach was to use regnal years—the reigning Roman emperor, for example, or the ruler of whichever kingdom was under discussion.

For example, it's easy to accept that GNU date doesn't claim to state the Jewish or Islamic month or year, nor the Japanese imperial regnal year, nor the Julian date that some Eastern Orthodox Christians continue to use for some religious purposes, nor the Roman year ab urbe condita.