Day in Rome that is timeless (4)

I believe the answer is:

ides

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'day in rome' is the definition.
(Roman calendar term, as in the Ides of March)

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

Can you help me to learn more?

(Other definitions for ides that I've seen before include "March day Caesar died" , "Day in Roman months" , "March day fatal to Julius Caesar" , "Caesar was killed in this side of March" , "Julius Caesar was killed during the . . . . of March" .)

I've seen this clue in The Times.

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