I think this should do it for you.

=SUMPRODUCT(--(DATEDIF(A1:A5,TODAY(),"y")<1))/COUNT(A1:A5)

A1:A5 is your range of dates.

This counts the number of dates that are less than 1 full year from
today's date and divides it by the total number of dates. Format your
cell as percentage.

Does that help?

Steve


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