Change a #NUM! to a text ?

So I am doing a table with "datedif" formulas and when a date is negative, it's gives the #NUM! error. What I would like to do is have the error becoming a text I chose , that would be "expired".

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I am a very very beginner in Google Sheet so please, be kind.

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Also, could I make the cell have different colors depending on the answer they give ?