[lukev]

Well it depends on if you have a type system that distinguishes between "date" types and "instant" types.

If you only have instants, then sure, you could do what you're saying, but the context in which the question was poised seems to imply that "dates" and "times" are separate.

And if you do have separate "date" and "instant" types, you lose the property of substitution: f(x) = f(y) if x = y for any arbitrary function f.