Select the cell you want to apply the condition to.
In the conditional format box, select "formula is"

type:
=today()-[DueDateCell] GreaterThanSymbol 365

where "[DueDateCell] is the cell where the due date is entered.

(This board removes the greater-than-symbol, so replace
"GreaterThanSymbol" with the correct mark

Set the formatting

Your done!


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