Can we add requirements for NanosecondsToDays to disallow bad return values, like for example:

1. Require the sign for result.[[Days]] and result.[[Nanoseconds]] matches the sign of the input nanoseconds.
2. Require that abs(result.[[Nanoseconds]]) is less than result.[[DayLength]].
3. Maybe also require that result.[[DayLength]] is less than some constant, for example nsPerDay * 2?