Unfortunately dated leap years certainly don't! (8)

I believe the answer is:

lackaday

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'unfortunately' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for lackaday that I've seen before include "unfortunately" , "How sad" , "cry of dismay" , "alas" .)

I've seen this clue in The Telegraph.

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