Access to date and time using fields and units.

This package expands on the base package to provide additional functionality for more powerful use cases. Support is included for:

- Units of date-time, such as years, months, days and hours
- Fields of date-time, such as month-of-year, day-of-week or hour-of-day
- Date-time adjustment functions
- Different definitions of weeks

#### Fields and Units

Dates and times are expressed in terms of fields and units. A unit is used to measure an amount of time, such as years, days or minutes. All units implement {@link org.threeten.bp.temporal.TemporalUnit}. The set of well known units is defined in {@link org.threeten.bp.temporal.ChronoUnit}, for example, {@link org.threeten.bp.temporal.ChronoUnit#DAYS}. The unit interface is designed to allow applications to add their own units.

A field is used to express part of a larger date-time, such as year, month-of-year or second-of-minute. All fields implement {@link org.threeten.bp.temporal.TemporalField}. The set of well known fields are defined in {@link org.threeten.bp.temporal.ChronoField}, for example, {@link org.threeten.bp.temporal.ChronoField#HOUR_OF_DAY}. An additional fields are defined by {@link org.threeten.bp.temporal.JulianFields}. The field interface is designed to allow applications to add their own fields.

This package provides tools that allow the units and fields of date and time to be accessed in a general way most suited for frameworks. {@link org.threeten.bp.temporal.Temporal} provides the abstraction for date time types that support fields. Its methods support getting the value of a field, creating a new date time with the value of a field modified, and extracting another date time type, typically used to extract the offset or time-zone.

One use of fields in application code is to retrieve fields for which there is no convenience method. For example, getting the day-of-month is common enough that there is a method on {@code LocalDate} called {@code getDayOfMonth()}. However for more unusual fields it is necessary to use the field. For example, {@code date.get(ChronoField.ALIGNED_WEEK_OF_MONTH)}. The fields also provide access to the range of valid values.

#### Adjustment

A key part of the date-time problem space is adjusting a date to a new, related value, such as the "last day of the month", or "next Wednesday". These are modeled as functions that adjust a base date-time. The functions implement {@link org.threeten.bp.temporal.TemporalAdjuster} and operate on {@link org.threeten.bp.temporal.Temporal}. A set of common functions are provided in {@link org.threeten.bp.temporal.TemporalAdjusters}. For example, to find the first occurrence of a day-of-week after a given date, use {@link org.threeten.bp.temporal.TemporalAdjusters#next(DayOfWeek)}, such as {@code date.with(next(MONDAY))}.

#### Weeks

Different locales have different definitions of the week. For example, in Europe the week typically starts on a Monday, while in the US it starts on a Sunday. The {@link org.threeten.bp.temporal.WeekFields} class models this distinction.

The ISO calendar system defines an additional week-based division of years. This defines a year based on whole Monday to Monday weeks. This is modeled in {@link org.threeten.bp.temporal.IsoFields}.