A date-based amount of time, such as '3 years, 4 months and 5 days' _in an arbitrary chronology, intended for advanced globalization use cases. !(p) This interface models a date-based amount of time _in a calendar system. While most calendar systems use years, months and days, some do not. Therefore, this interface operates solely _in terms of a set of supported units that are defined by the {@code Chronology}. The set of supported units is fixed for a given chronology. The amount of a supported unit may be set to zero. !(p) The period is modeled as a directed amount of time, meaning that individual parts of the period may be negative.