Yeah, Celsius is awkward because it's basically used as both an actual temperature and as an interval depending on context, and mixing them is bad news. A similar sort of distinction often shows up in datetime libraries between an instant and a duration/timespan, but I'm not sure it's worth doing in a unit library. Just forcing kelvin for intervals probably isn't too onerous for most uses, and I can't think of any unit it would really apply to other than °C/F, whosoever dares mix units of measure with time zones notwithstanding.