Periods of world revolution! (5)

I believe the answer is:

years

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'periods' is the definition.
(year is a kind of period)

I don't understand how the remainder of the clue works.

(Other definitions for years that I've seen before include "Periods of time" , "Age" , "12 month periods" , "Long periods" , "Ages" .)

I've seen this clue in The Guardian.

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