Provides support for dates, times, time zones, durations, intervals, and partials. This package aims to fully replace the Java Date, Calendar, and TimeZone classes. This implementation covers both the Gregorian/Julian calendar system and the ISO8601 standard. Additional calendar systems and extensions can be created as well.

The ISO8601 standard is the international standard for dates, times, durations, and intervals. It defines text representations, the first day of the week as Monday, and the first week in a year as having a Thursday in it. This standard is being increasingly used in computer interchange and is the agreed format for XML. For most uses, the ISO standard is the same as Gregorian, and is thus the preferred format.

Interfaces

The main API concepts are defined by interfaces:

These define the public interface to dates, times, periods, intervals and durations. As with java.util.Date and Calendar, the design is millisecond based with an epoch of 1970-01-01. This should enable easy conversions.

Implementations

The basic implementation of the ReadableInstant interface is Instant. This is a simple immutable class that stores the millisecond value and integrates with Java Date and Calendar. The class follows the definition of the millisecond instant fully, thus it references the ISO-8601 calendar system and UTC time zone. If you are dealing with an instant in time but do not know, or do not want to specify, which calendar system it refers to, then you should use this class.

The main implementation class for datetimes is the DateTime class. This implements the ReadableDateTime interface, providing convenient methods to access the fields of the datetime. Conversion methods allow integration with the Java Date and Calendar classes.

Like Instant, DateTime is immutable, and it can be used safely in a multi-threaded environment. In order to be fully immutable, key clases are declared as final. Abstract superclasses are provided should you need to define your own implementations.

The concrete implementations of the ReadWritable... interfaces are named the same as their immutable counterparts, but with a "Mutable" prefix. For example, MutableDateTime implements ReadWritableDateTime, making datetime editing easy. Note that it is possible to use the immutable DateTime for modifying datetimes, however each modification method returns a new instance of DateTime.

Interface usage

The interfaces in Joda-Time are not designed to operate in the same way as those in the Java Collections Framework (List/Map/Set etc). The Joda-Time interfaces represent a core subset of the functionality available via the actual classes. Thus, much of the work of an application will probably use methods on the class, not on the interface. Your application must determine whether it should define dates in terms of the interfaces, or in terms of the classes.

The interfaces provide simple methods to access an instance of the immutable class, which is implemented either via typecast or object creation. Thus, if you hold a reference to a ReadableInstant, and you call the method toDateTime(), the same instance will be returned (typecast) if it already was a DateTime.

Chronologies and Fields

In order to enable the package to be easily extended, each field of the datetime, such as the month, is calculated by an implementation of DateTimeField. Likewise, duration fields are calculated by specialized DurationField instances. If desired, users can write their own implementations to retrieve an unusual field from the millisecond value.

The datetime and duration fields that together represent a calendar system are grouped into a Chronology. The chronology represents all the information to convert from a millisecond value to human understandable fields in a specific calendar system. Chronologies are provided for ISO, Gregorian/Julian (GJ), Coptic and Buddhist. More implementations are sought from the community.

The chronology and field classes are singletons. This design results in a low overhead on the date and time classes. The Java Calendar class performs poorly because it has many internal fields that are constantly kept in sync. This design only calculates fields when required, resulting in lightweight and simple date time classes.

When reviewing the library for the first time, it is easy to mistake the number of classes with complexity. The library is in fact clearly divided between user packages and implementation packages in the javadoc. Most users will should not need to be concerned with the back-end implementation.

Partials

Partials are like instants, except they do not completely specify a point in time. The main interface is ReadablePartial, and implementations include TimeOfDay (contains hour, minute, second, and millis) and YearMonthDay.

The API of a partial is remarkably similar to an instant, however there are internal differences. DateTime and Instant store a long millisecond value internally and calculate field values on demand. The partial classes work the other way around, storing field values internally and providing a means to resolve to a millisecond value on demand.

Since a partial does not represent a specific point in time, it must be resolved to create a full instant. For example, a TimeOfDay might represent 12:30. To convert to a DateTime requires the specification of a date, and is performed using the resolve methods of ReadablePartial. For example, 

    new TimeOfDay("12:30").resolveDateTime(new DateTime("2004-06-07T06:00"));
equals 
    new DateTime("2004-06-07T12:30");

Formatting

Formatting is provided by the format subpackage. Comprehensive support is provided for outputting dates and times in multiple formats. A pattern similar to Java SimpleDateFormat can be used, but a more advanced programmatic technique is available via the builder classes.