A time period is divided into a number of fields, such as hours and seconds. Which fields are supported is defined by the PeriodType class. The default is the standard period type, which supports years, months, weeks, days, hours, minutes, seconds and millis.
When this time period is added to an instant, the effect is of adding each field in turn. As a result, this takes into account daylight savings time. Adding a time period of 1 day to the day before daylight savings starts will only add 23 hours rather than 24 to ensure that the time remains the same. If this is not the behaviour you want, then see {@link Duration}.
The definition of a period also affects the equals method. A period of 1 day is not equal to a period of 24 hours, nor 1 hour equal to 60 minutes. This is because periods represent an abstracted definition of a time period (eg. a day may not actually be 24 hours, it might be 23 or 25 at daylight savings boundary). To compare the actual duration of two periods, convert both to durations using toDuration, an operation that emphasises that the result may differ according to the date you choose.
Period is thread-safe and immutable, provided that the PeriodType is as well. All standard PeriodType classes supplied are thread-safe and immutable. @author Brian S O'Neill @author Stephen Colebourne @since 1.0 @see MutablePeriod
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