Should epoch parameter of eras also have an era?

Currently era() assumes the epoch parameter is a year in the Common Era, but we could make this itself a yr object (defaulting to CE).

Start by locating the era() implementation and the yr object definition in the R package. Read how the epoch parameter is currently interpreted and check existing tests for era() and yr values. Done means epoch accepts a yr object while retaining the current CE default and behavior.

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