Question

@Vincent-Lyon.Fr
Observation: I observe that a sentence "Tag und Nacht ist dein" exists.
Question: Why isn't it "sind" instead of "ist"?
Hypothesis: I think that it may be old German grammar.

Answer 1

Probably. Tag-und-Nacht was considered as one word.
I think "Day-and-night is yours" would not be incorrect even in English.

Answer 2

But I also found a sentence like this: "Getrost ist mir mein Herz und Sinn"