A standard set of date periods units. !(p) This set of units provide unit-based access to manipulate a date, time or date-time. The standard set of units can be extended by implementing {@link TemporalUnit}. !(p) These units are intended to be applicable _in multiple calendar systems. For example, most non-ISO calendar systems define units of years, months and days, just with slightly different rules. The documentation of each unit explains how it operates.