This class uses a Chronology internally. The Chronology determines how the
millisecond instant value is converted into the date time fields.
The default Chronology is ISOChronology which is the agreed
international standard and compatable with the modern Gregorian calendar.
Each individual field can be queried in two ways:
DateTime is thread-safe and immutable, provided that the Chronology is as well.
All standard Chronology classes supplied are thread-safe and immutable.