I agree on reflection that a single nanoseconds integral value makes more sense. This then requires refactoring of the existing code so that existing tests continue to pass using a microsecond property.

Code using ONLY nanoseconds is a disjoint case, for which new tests will be required. It clearly cannot be expected to be backwards compatible with pre-implementation versions.

Does it make sense to define behaviour for cases where the user attempts to MIX microseconds and nanoseconds? One validation I would suggest if so is that in the presence of a microseconds specification a constraint of 0 <= nanoseconds < 1000 must be imposed.