A date-based amount of time _in the ISO-8601 calendar system, such as '2 years, 3 months and 4 days'. !(p) This class models a quantity or amount of time _in terms of years, months and days. See {@link Duration} for the time-based equivalent to this class. !(p) Durations and periods differ _in their treatment of daylight savings time when added to {@link ZonedDateTime}. A {@code Duration} will add an exact number of seconds, thus a duration of one day is always exactly 24 hours. By contrast, a {@code Period} will add a conceptual day, trying to maintain the local time. !(p) For example, consider adding a period of one day and a duration of one day to 18:00 on the evening before a daylight savings gap. The {@code Period} will add the conceptual day and result _in a {@code ZonedDateTime} at 18:00 the following day. By contrast, the {@code Duration} will add exactly 24 hours, resulting _in a {@code ZonedDateTime} at 19:00 the following day (assuming a one hour DST gap). !(p) The supported units of a period are {@link ChronoUnit#YEARS YEARS}, {@link ChronoUnit#MONTHS MONTHS} and {@link ChronoUnit#DAYS DAYS}. All three fields are always present, but may be set to zero. !(p) The ISO-8601 calendar system is the modern civil calendar system used today _in most of the world. It is equivalent to the proleptic Gregorian calendar system, _in which today's rules for leap years are applied for all time. !(p) The period is modeled as a directed amount of time, meaning that individual parts of the period may be negative.