diff --git a/README.md b/README.md
index ee9e0a0..a9017b4 100644
--- a/README.md
+++ b/README.md
@@ -6,7 +6,9 @@
 [![Code Coverage](https://img.shields.io/coveralls/ICanBoogie/DateTime.svg)](https://coveralls.io/r/ICanBoogie/DateTime)
 [![Packagist](https://img.shields.io/packagist/dm/icanboogie/datetime.svg?maxAge=2592000)](https://packagist.org/packages/icanboogie/datetime)
 
-This package extends the features of PHP [DateTime](http://www.php.net/manual/en/class.datetime.php)
+This package extends the features of PHP
+[DateTime](http://www.php.net/manual/en/class.datetime.php),
+[DateTimeImmutable](http://www.php.net/manual/en/class.datetimeimmutable.php),
 and [DateTimeZone](http://www.php.net/manual/en/class.datetimezone.php) classes to ease the
 handling of times, time zones and time zone locations. Getting the UTC or local representation of
 a time, formatting the time to a predefined format, accessing common properties such as day, month,
@@ -83,11 +85,11 @@ The implementation of the [DateTime][] class is vastly inspired by Ruby's
 
 ## DateTime is a value object
 
-Starting from v2.0, date time instances are immutable [value
-objects](https://en.wikipedia.org/wiki/Value_object). A class to create mutable date time instances
-is available, but it's important to remember the difference with PHP implementation. Thus,
-[ICanBoogie\DateTime][] extends [\DateTimeImmutable][] and [ICanBoogie\MutableDateTime][] extends
-[\DateTime][].
+Starting from v2.0, date time instances are immutable [value objects][]. A class to create mutable
+date time instances is available, but it's important to remember the difference with PHP
+implementation. Thus, [ICanBoogie\DateTime][] extends [\DateTimeImmutable][] and
+[ICanBoogie\MutableDateTime][] extends [\DateTime][]. Alose [ICanBoogie\Contract\DateTime], which is
+implemented by both [DateTime][] and [MutableDateTime][], extends [\DateTimeInterface][].
 
 You can switch from immutable to mutable instances using the `mutable` and `immutable` properties,
 or the static `from()` method:
@@ -485,16 +487,17 @@ The package is continuously tested by [Travis CI](http://about.travis-ci.org/).
 
 
 
-[ICanBoogie/CLDR]:            https://github.com/ICanBoogie/CLDR
-[JsonSerializable interface]: http://php.net/manual/en/class.jsonserializable.php
-[documentation]:              http://api.icanboogie.org/datetime/latest/
-[\DateTime]:                  http://php.net/manual/en/class.datetime.php
-[\DateTimeImmutable]:         http://php.net/manual/en/class.datetimeimmutable.php
-[DateTime]:                   http://api.icanboogie.org/datetime/2.0/class-ICanBoogie.DateTime.html
-[ICanBoogie\DateTime]:        http://api.icanboogie.org/datetime/2.0/class-ICanBoogie.DateTime.html
-[ICanBoogie\MutableDateTime]: http://api.icanboogie.org/datetime/2.0/class-ICanBoogie.MutableDateTime.html
-[MutableDateTime]:            http://api.icanboogie.org/datetime/2.0/class-ICanBoogie.MutableDateTime.html
-[TimeZone]:                   http://api.icanboogie.org/datetime/2.0/class-ICanBoogie.TimeZone.html
-[TimeZoneLocation]:           http://api.icanboogie.org/datetime/2.0/class-ICanBoogie.TimeZoneLocation.html
-[PropertyNotDefined]:         http://api.icanboogie.org/common/1.2/class-ICanBoogie.PropertyNotDefined.html
-[PropertyNotWritable]:        http://api.icanboogie.org/common/1.2/class-ICanBoogie.PropertyNotWritable.html
+[ICanBoogie/CLDR]:              https://github.com/ICanBoogie/CLDR
+[JsonSerializable interface]:   http://php.net/manual/en/class.jsonserializable.php
+[documentation]:                http://api.icanboogie.org/datetime/latest/
+[\DateTime]:                    http://php.net/manual/en/class.datetime.php
+[\DateTimeImmutable]:           http://php.net/manual/en/class.datetimeimmutable.php
+[\DateTimeInterface]:           http://php.net/manual/en/class.datetimeinterface.php
+[DateTime]:                     http://api.icanboogie.org/datetime/2.0/class-ICanBoogie.DateTime.html
+[ICanBoogie\Contract\DateTime]: http://api.icanboogie.org/datetime/2.0/class-ICanBoogie.Contract.DateTime.html
+[ICanBoogie\DateTime]:          http://api.icanboogie.org/datetime/2.0/class-ICanBoogie.DateTime.html
+[ICanBoogie\MutableDateTime]:   http://api.icanboogie.org/datetime/2.0/class-ICanBoogie.MutableDateTime.html
+[MutableDateTime]:              http://api.icanboogie.org/datetime/2.0/class-ICanBoogie.MutableDateTime.html
+[TimeZone]:                     http://api.icanboogie.org/datetime/2.0/class-ICanBoogie.TimeZone.html
+[TimeZoneLocation]:             http://api.icanboogie.org/datetime/2.0/class-ICanBoogie.TimeZoneLocation.html
+[value objects]:                https://en.wikipedia.org/wiki/Value_object
diff --git a/lib/Contract/DateTime.php b/lib/Contract/DateTime.php
new file mode 100644
index 0000000..552695f
--- /dev/null
+++ b/lib/Contract/DateTime.php
@@ -0,0 +1,149 @@
+<?php
+
+namespace ICanBoogie\Contract;
+
+/**
+ * Interface for ICanBoogie's mutable and immutable DateTime.
+ */
+interface DateTime extends \DateTimeInterface
+{
+	/**
+	 * DB (example: 2013-02-03 20:59:03)
+	 *
+	 * @var string
+	 */
+	const DB = 'Y-m-d H:i:s';
+
+	/**
+	 * Number (example: 20130203205903)
+	 *
+	 * @var string
+	 */
+	const NUMBER = 'YmdHis';
+
+	/**
+	 * Date (example: 2013-02-03)
+	 *
+	 * @var string
+	 */
+	const DATE = 'Y-m-d';
+
+	/**
+	 * Time (example: 20:59:03)
+	 *
+	 * @var string
+	 */
+	const TIME = 'H:i:s';
+
+	/**
+	 * Creates a {@link DateTime} instance from a source.
+	 *
+	 * <pre>
+	 * <?php
+	 *
+	 * use ICanBoogie\DateTime;
+	 *
+	 * DateTime::from(new \DateTime('2001-01-01 01:01:01', new \DateTimeZone('Europe/Paris')));
+	 * DateTime::from('2001-01-01 01:01:01', 'Europe/Paris');
+	 * DateTime::from('now');
+	 * </pre>
+	 *
+	 * @param mixed $source
+	 * @param mixed $timezone The time zone to use to create the time. The value is ignored if the
+	 * source is an instance of {@link \DateTime}.
+	 *
+	 * @return static
+	 */
+	static public function from($source, $timezone = null);
+
+	/**
+	 * Returns an instance representing an empty date ("0000-00-00").
+	 *
+	 * <pre>
+	 * <?php
+	 *
+	 * use ICanBoogie\DateTime;
+	 *
+	 * $d = DateTime::none();
+	 * $d->is_empty;                      // true
+	 * $d->timezone->name;                // "UTC"
+	 *
+	 * $d = DateTime::none('Asia/Tokyo');
+	 * $d->is_empty;                      // true
+	 * $d->timezone->name;                // "Asia/Tokio"
+	 * </pre>
+	 *
+	 * @param \DateTimeZone|string $timezone The time zone in which the empty date is created.
+	 * Defaults to "UTC".
+	 *
+	 * @return DateTime
+	 */
+	static public function none($timezone = 'utc');
+
+	/**
+	 * Returns an instance with the current local time and the local time zone.
+	 *
+	 * **Note:** Subsequent calls return equal times, event if they are minutes apart. _now_
+	 * actually refers to the `REQUEST_TIME` or, if it is now available, to the first time
+	 * the method was invoked.
+	 *
+	 * @return static
+	 */
+	static public function now();
+
+	/**
+	 * Returns an instance with the current local time and the local time zone.
+	 *
+	 * **Note:** Subsequent calls may return different times.
+	 *
+	 * @return static
+	 */
+	static public function right_now();
+
+	/**
+	 * Modifies the properties of the instance according to the options.
+	 *
+	 * The following properties can be updated: {@link $year}, {@link $month}, {@link $day},
+	 * {@link $hour}, {@link $minute} and {@link $second}.
+	 *
+	 * Note: Values exceeding ranges are added to their parent values.
+	 *
+	 * <pre>
+	 * <?php
+	 *
+	 * use ICanBoogie\DateTime;
+	 *
+	 * $time = new DateTime('now');
+	 * $time->change([ 'year' => 2000, 'second' => 0 ]);
+	 * </pre>
+	 *
+	 * @param array $options
+	 * @param bool $cascade If `true`, time options (`hour`, `minute`, `second`) reset
+	 * cascading, so if only the hour is passed, then minute and second is set to 0. If the hour
+	 * and minute is passed, then second is set to 0.
+	 *
+	 * @return $this
+	 */
+	public function change(array $options, $cascade = false);
+
+	/**
+	 * Returns a new instance with changes properties.
+	 *
+	 * @param array $options
+	 * @param bool $cascade
+	 *
+	 * @return DateTime
+	 */
+	public function with(array $options, $cascade = false);
+
+	/**
+	 * Returns a localized instance.
+	 *
+	 * @param string $locale
+	 *
+	 * @return mixed
+	 *
+	 * @throws \LogicException if the instance cannot be localized.
+	 */
+	public function localize($locale = 'en');
+}
diff --git a/lib/DateTime.php b/lib/DateTime.php
index 716b555..5987ce1 100644
--- a/lib/DateTime.php
+++ b/lib/DateTime.php
@@ -11,8 +11,6 @@
 
 namespace ICanBoogie;
 
-use ICanBoogie\DateTime\AdditionalFormats;
-
 /**
  * Representation of a date and time.
  *
@@ -102,7 +100,7 @@
  *
  * @see http://en.wikipedia.org/wiki/ISO_8601
  */
-class DateTime extends \DateTimeImmutable implements \JsonSerializable, AdditionalFormats
+class DateTime extends \DateTimeImmutable implements \JsonSerializable, Contract\DateTime
 {
 	use DateTime\Shared;
 	use DateTime\Readers;
@@ -130,13 +128,7 @@ class DateTime extends \DateTimeImmutable implements \JsonSerializable, Addition
 	static public $localizer = null;
 
 	/**
-	 * Returns an instance with the current local time and the local time zone.
-	 *
-	 * **Note:** Subsequent calls return equal times, event if they are minutes apart. _now_
-	 * actually refers to the `REQUEST_TIME` or, if it is now available, to the first time
-	 * the method was invoked.
-	 *
-	 * @return static
+	 * @inheritdoc
 	 */
 	static public function now()
 	{
@@ -161,12 +153,7 @@ public function __set($property, $value)
 	}
 
 	/**
-	 * Instantiate a new instance with changes properties.
-	 *
-	 * @param array $options
-	 * @param bool $cascade
-	 *
-	 * @return DateTime
+	 * @inheritdoc
 	 */
 	public function with(array $options, $cascade = false)
 	{
diff --git a/lib/DateTime/AdditionalFormats.php b/lib/DateTime/AdditionalFormats.php
deleted file mode 100644
index 8d90349..0000000
--- a/lib/DateTime/AdditionalFormats.php
+++ /dev/null
@@ -1,46 +0,0 @@
-<?php
-
-/*
- * This file is part of the ICanBoogie package.
- *
- * (c) Olivier Laviale <olivier.laviale@gmail.com>
- *
- * For the full copyright and license information, please view the LICENSE
- * file that was distributed with this source code.
- */
-
-namespace ICanBoogie\DateTime;
-
-/**
- *
- */
-interface AdditionalFormats
-{
-	/**
-	 * DB (example: 2013-02-03 20:59:03)
-	 *
-	 * @var string
-	 */
-	const DB = 'Y-m-d H:i:s';
-
-	/**
-	 * Number (example: 20130203205903)
-	 *
-	 * @var string
-	 */
-	const NUMBER = 'YmdHis';
-
-	/**
-	 * Date (example: 2013-02-03)
-	 *
-	 * @var string
-	 */
-	const DATE = 'Y-m-d';
-
-	/**
-	 * Time (example: 20:59:03)
-	 *
-	 * @var string
-	 */
-	const TIME = 'H:i:s';
-}
diff --git a/lib/DateTime/Shared.php b/lib/DateTime/Shared.php
index 4b63e53..a53fe68 100644
--- a/lib/DateTime/Shared.php
+++ b/lib/DateTime/Shared.php
@@ -72,23 +72,7 @@ abstract public function setDate($year, $month, $day);
 	abstract public function setTime($hour, $minute, $second = null);
 
 	/**
-	 * Creates a {@link DateTime} instance from a source.
-	 *
-	 * <pre>
-	 * <?php
-	 *
-	 * use ICanBoogie\DateTime;
-	 *
-	 * DateTime::from(new \DateTime('2001-01-01 01:01:01', new \DateTimeZone('Europe/Paris')));
-	 * DateTime::from('2001-01-01 01:01:01', 'Europe/Paris');
-	 * DateTime::from('now');
-	 * </pre>
-	 *
-	 * @param mixed $source
-	 * @param mixed $timezone The time zone to use to create the time. The value is ignored if the
-	 * source is an instance of {@link \DateTime}.
-	 *
-	 * @return DateTime
+	 * @inheritdoc
 	 */
 	static public function from($source, $timezone = null)
 	{
@@ -106,26 +90,7 @@ static public function from($source, $timezone = null)
 	}
 
 	/**
-	 * Returns an instance representing an empty date ("0000-00-00").
-	 *
-	 * <pre>
-	 * <?php
-	 *
-	 * use ICanBoogie\DateTime;
-	 *
-	 * $d = DateTime::none();
-	 * $d->is_empty;                      // true
-	 * $d->timezone->name;                // "UTC"
-	 *
-	 * $d = DateTime::none('Asia/Tokyo');
-	 * $d->is_empty;                      // true
-	 * $d->timezone->name;                // "Asia/Tokio"
-	 * </pre>
-	 *
-	 * @param \DateTimeZone|string $timezone The time zone in which the empty date is created.
-	 * Defaults to "UTC".
-	 *
-	 * @return DateTime
+	 * @inheritdoc
 	 */
 	static public function none($timezone = 'utc')
 	{
@@ -133,11 +98,7 @@ static public function none($timezone = 'utc')
 	}
 
 	/**
-	 * Returns an instance with the current local time and the local time zone.
-	 *
-	 * **Note:** Subsequent calls may return different times.
-	 *
-	 * @return static
+	 * @inheritdoc
 	 */
 	static public function right_now()
 	{
@@ -254,28 +215,7 @@ public function format($format)
 	}
 
 	/**
-	 * Modifies the properties of the instance according to the options.
-	 *
-	 * The following properties can be updated: {@link $year}, {@link $month}, {@link $day},
-	 * {@link $hour}, {@link $minute} and {@link $second}.
-	 *
-	 * Note: Values exceeding ranges are added to their parent values.
-	 *
-	 * <pre>
-	 * <?php
-	 *
-	 * use ICanBoogie\DateTime;
-	 *
-	 * $time = new DateTime('now');
-	 * $time->change([ 'year' => 2000, 'second' => 0 ]);
-	 * </pre>
-	 *
-	 * @param array $options
-	 * @param bool $cascade If `true`, time options (`hour`, `minute`, `second`) reset
-	 * cascading, so if only the hour is passed, then minute and second is set to 0. If the hour
-	 * and minute is passed, then second is set to 0.
-	 *
-	 * @return $this
+	 * @inheritdoc
 	 */
 	public function change(array $options, $cascade = false)
 	{
@@ -340,13 +280,7 @@ public function change(array $options, $cascade = false)
 	}
 
 	/**
-	 * Returns a localized instance.
-	 *
-	 * @param string $locale
-	 *
-	 * @return mixed
-	 *
-	 * @throws \LogicException if {@link $localizer} is not defined.
+	 * @inheritdoc
 	 */
 	public function localize($locale = 'en')
 	{
diff --git a/lib/MutableDateTime.php b/lib/MutableDateTime.php
index 773484e..2f66cac 100644
--- a/lib/MutableDateTime.php
+++ b/lib/MutableDateTime.php
@@ -11,8 +11,6 @@
 
 namespace ICanBoogie;
 
-use ICanBoogie\DateTime\AdditionalFormats;
-
 /**
  * @property int $year Year.
  * @property int $month Month of the year.
@@ -29,19 +27,13 @@
  *
  * @method $this change(array $options, $cascade = false)
  */
-class MutableDateTime extends \DateTime implements \JsonSerializable, AdditionalFormats
+class MutableDateTime extends \DateTime implements \JsonSerializable, Contract\DateTime
 {
 	use DateTime\Shared;
 	use DateTime\Readers;
 
 	/**
-	 * Returns an instance with the current local time and the local time zone.
-	 *
-	 * **Note:** Subsequent calls return equal times, event if they are minutes apart. _now_
-	 * actually refers to the `REQUEST_TIME` or, if it is now available, to the first time
-	 * the method was invoked.
-	 *
-	 * @return static
+	 * @inheritdoc
 	 */
 	static public function now()
 	{
@@ -124,14 +116,8 @@ public function __set($property, $value)
 		throw new \LogicException("Property is not defined: $property.");
 	}
 
-
 	/**
-	 * Instantiate a new instance with changes properties.
-	 *
-	 * @param array $options
-	 * @param bool $cascade
-	 *
-	 * @return MutableDateTime
+	 * @inheritdoc
 	 */
 	public function with(array $options, $cascade = false)
 	{