Correct plural form of a noun preceded by "zero"

When using zero as a quantifier, is it correct to use the singular form on the object of the quantifier, or the plural form?

It sounds confusing when I put it that way, but what I mean is: Which is correct?

• Your password expires in 0 days.
• Your password expires in 0 day.

Essentially I suppose I'm asking, does "singular" mean "one" or "the opposite of more than one", as zero is not "plural" in the traditional "more than one" sense?

I'm pretty sure "days" sounds correct, but I can't be sure.

In English, every number that is not 1 is considered plural. The correct sentence is the first you wrote.

Your password expires in 0 days.

For your specific example, since you're doing processing to check for != 1 day, I'd recommend spending the few extra lines of code to produce "Your password expires today", "Your password expires tomorrow", or "your password expires in X days".

In the general case, 0 does count (rather non-intuitively) as a plural number.