Ah, but the surprising date behavior that @itaiferber called out wasn't due to Samoa using a non-Gregorian calendar and deciding to align with the Gregorian calendar, it was due to a decision to change the time zone offset used by their geo-political region from UTC–11 to UTC+13.
Of course, if you go so far as to say, well, given a fixed calendar, fixed time zone, fixed location, for a fixed range of the calendar, can't we perform determinate calculations on such 'timeless' dates, then, perhaps (though I'm prepared for Dave to show me the exception 
). But that's defining away a whole lot of complexity and I'd probably need substantial convincing that such specific utilities are of sufficient use to deserve a place as general API surface! IMO better to provide the general calendrical facilities and leave it to users to specify for their narrow cases "I am okay with 'next day' assuming a fixed calendar, time zone, location, and date range for my use".