They mark time (9)

I believe the answer is:

calendars

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'they' is the definition.
Both the answer and definition are plural nouns.
Perhaps they are linked in a way I don't understand?

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

Can you help me to learn more?

(Other definitions for calendars that I've seen before include "Lists of days, weeks, months" , "Tabular arrays of days" .)

I've seen this clue in The Irish News and The Globe and Mail.

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