I would translate it as Unvergänglich:

Mondschein und Liebeslieder sind unvergänglich

It sounds more natural and poetic than "niemals vergangen" and generally has a quite positive connotation.

More literal translations of "out-of-date" are:

• altmodisch (belittling connotation),
• veraltet (neutral),
• altbacken (derogatory).

I would translate never out of date as unvergänglich:

Mondschein und Liebeslieder sind unvergänglich

It sounds more natural and poetic than "niemals vergangen" and generally has a quite positive connotation.

More literal translations of "out-of-date" are:

• altmodisch (belittling connotation),
• veraltet (neutral),
• altbacken (derogatory).

I would translate it as Unvergänglich:

Mondschein und Liebeslieder sind unvergänglich

It sounds more natural and poetic than "niemals vergangen".

Do you want/have to preserve rhyme and rhythm?

I would translate it as Unvergänglich:

Mondschein und Liebeslieder sind unvergänglich

It sounds more natural and poetic than "niemals vergangen" and generally has a quite positive connotation.

More literal translations of "out-of-date" are:

• altmodisch (belittling connotation),
• veraltet (neutral),
• altbacken (derogatory).