The docs for the artist-evolution-activity endpoints document the time_unit value in function of the requested stats range.

But it only lists the original set of stat range values.

In most cases, the results match one of the original ranges; half_yearly matches year (month name), this_month/this_week/this_year match the month/week/year (day of month/name of day of week/name of month).
However, quarter does not, as you might expect, provide the month name (or something new like "Q1"/"Q2"/...) - it has the year instead.

What's worse, the sitewide version of the endpoint seems to have time_unit as an integer instead of a string for quarter and this_month (possibly all_time and month too, but those currently give me an empty result set), which is likely to break clients.

(Note: the use of English weekday and month names is a bit unfortunate from an international use perspective; what's more, if that had not been chosen, time_unit could just be an integer and take less space in the payload.)