Those are the expectations of the standard library, not of somebodies code outside of that scope. It should be expected though that any library code that could be used outside of itself conforms to these requirements in order to make it easier on those using it. Internally however this is less important (though, something to strive for)

So, unless you are writing a library that passes this back to a user, a programmer, this isn't an issue.

You should however look into other possibilities, ensure it's not possible to implement a .now() function, if none found document that .now() is not implemented along with an explanation as to why, and if possible make sure a good error is presented if it's use is attempted.

To answer more concisely it depends on context. If it's internal, maintainable, and easily explainable then it's fine. If it can become external, isn't explainable, isn't maintainable, then no. Find a different way.