In the context of the calculator, you already know the start and end times, and it shows the duration in terms of days (which is unambiguous given that we know

In the context of the calculator, you already know the start and end times, and it shows the duration in terms of days (which is unambiguous given that we know the interval when the duration takes place). I think that it's clear that the duration in terms of months, weeks and hours is there just to put it in more easily graspable terms, at the expense of precision.