diff -u b/core/core.services.yml b/core/core.services.yml
--- b/core/core.services.yml
+++ b/core/core.services.yml
@@ -537,7 +537,7 @@
     arguments: ['@module_handler']
   date:
     class: Drupal\Core\Datetime\Date
-    arguments: ['@entity.manager', '@language_manager', '@string_translation']
+    arguments: ['@entity.manager', '@language_manager']
   feed.bridge.reader:
     class: Drupal\Component\Bridge\ZfExtensionManagerSfContainer
     calls:
@@ -555,7 +555,7 @@
     arguments: ['@module_handler']
   date:
     class: Drupal\Core\Datetime\Date
-    arguments: ['@entity.manager', '@language_manager']
+    arguments: ['@entity.manager', '@language_manager', '@string_translation']
   feed.bridge.reader:
     class: Drupal\Component\Bridge\ZfExtensionManagerSfContainer
     calls:
diff -u b/core/includes/common.inc b/core/includes/common.inc
--- b/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -919,8 +919,7 @@
  * @see t()
  * @see format_string()
  *
- * @deprecated as of Drupal 8.0. Use the string_translation service formatPlural
- *   method.
+ * @deprecated as of Drupal 8.0. Use \Drupal::translation()->formatPlural()
  */
 function format_plural($count, $singular, $plural, array $args = array(), array $options = array()) {
   return \Drupal::translation()->formatPlural($count, $singular, $plural, $args, $options);
@@ -1002,7 +1001,7 @@
  * @return
  *   A translated string representation of the interval.
  *
- * @deprecated as of Drupal 8.0. Use the date service formatInterval method.
+ * @deprecated as of Drupal 8.0. Use \Drupal::service('date')->formatInterval().
  */
 function format_interval($interval, $granularity = 2, $langcode = NULL) {
   return \Drupal::service('date')->formatInterval($interval, $granularity, $langcode);
diff -u b/core/lib/Drupal/Core/Datetime/Date.php b/core/lib/Drupal/Core/Datetime/Date.php
--- b/core/lib/Drupal/Core/Datetime/Date.php
+++ b/core/lib/Drupal/Core/Datetime/Date.php
@@ -46,8 +46,9 @@
   /**
    * Contains the different date interval units.
    *
-   * This array is keyed by strings representating the unit (like 1 year|@count
-   * years) and with the amount of values of the unit in seconds.
+   * This array is keyed by strings representing the unit (e.g.
+   * '1 year|@count years') and with the amount of values of the unit in
+   * seconds.
    *
    * @var array
    */
@@ -58,7 +59,7 @@
     '1 day|@count days' => 86400,
     '1 hour|@count hours' => 3600,
     '1 min|@count min' => 60,
-    '1 sec|@count sec' => 1
+    '1 sec|@count sec' => 1,
   );
 
   /**
@@ -168,7 +169,7 @@
     foreach ($this->units as $key => $value) {
       $key = explode('|', $key);
       if ($interval >= $value) {
-        $output .= ($output ? ' ' : '') . format_plural(floor($interval / $value), $key[0], $key[1], array(), array('langcode' => $langcode));
+        $output .= ($output ? ' ' : '') . $this->stringTranslation->formatPlural(floor($interval / $value), $key[0], $key[1], array(), array('langcode' => $langcode));
         $interval %= $value;
         $granularity--;
       }
diff -u b/core/lib/Drupal/Core/StringTranslation/TranslationInterface.php b/core/lib/Drupal/Core/StringTranslation/TranslationInterface.php
--- b/core/lib/Drupal/Core/StringTranslation/TranslationInterface.php
+++ b/core/lib/Drupal/Core/StringTranslation/TranslationInterface.php
@@ -79,5 +79,5 @@
    * @see format_string()
    */
-  function formatPlural($count, $singular, $plural, array $args = array(), array $options = array());
+  public function formatPlural($count, $singular, $plural, array $args = array(), array $options = array());
 
 }
diff -u b/core/lib/Drupal/Core/StringTranslation/TranslationManager.php b/core/lib/Drupal/Core/StringTranslation/TranslationManager.php
--- b/core/lib/Drupal/Core/StringTranslation/TranslationManager.php
+++ b/core/lib/Drupal/Core/StringTranslation/TranslationManager.php
@@ -129,7 +129,7 @@
   /**
    * {@inheritdoc}
    */
-  function formatPlural($count, $singular, $plural, array $args = array(), array $options = array()) {
+  public function formatPlural($count, $singular, $plural, array $args = array(), array $options = array()) {
     $args['@count'] = $count;
     // Join both forms to search a translation.
     $tranlatable_string = implode(LOCALE_PLURAL_DELIMITER, array($singular, $plural));
only in patch2:
unchanged:
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Datetime/DateTest.php
@@ -0,0 +1,143 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\Core\Datetime\DateTest.
+ */
+
+namespace Drupal\Tests\Core\Datetime;
+
+use Drupal\Core\Datetime\Date;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * Tests the date service.
+ *
+ * @group Drupal
+ *
+ * @see \Drupal\Core\Datetime\Date
+ */
+class DateTest extends UnitTestCase {
+
+  /**
+   * The mocked entity manager.
+   *
+   * @var \Drupal\Core\Entity\EntityManager|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $entityManager;
+
+  /**
+   * The mocked language manager.
+   *
+   * @var \Drupal\Core\Language\LanguageManager|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $languageManager;
+
+  /**
+   * The mocked string translation.
+   *
+   * @var \Drupal\Core\StringTranslation\TranslationInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $stringTranslation;
+
+  /**
+   * The tested date service class.
+   *
+   * @var \Drupal\Core\Datetime\Date
+   */
+  protected $date;
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getInfo() {
+    return array(
+      'name' => 'Date service test.',
+      'description' => 'Tests the date service.',
+      'group' => 'System'
+    );
+  }
+
+  protected function setUp() {
+    $this->entityManager = $this->getMockBuilder('Drupal\Core\Entity\EntityManager')
+      ->disableOriginalConstructor()
+      ->getMock();
+    $this->languageManager = $this->getMockBuilder('Drupal\Core\Language\LanguageManager')
+      ->disableOriginalConstructor()
+      ->getMock();
+    $this->stringTranslation = $this->getMock('Drupal\Core\StringTranslation\TranslationInterface');
+
+
+
+    $this->date = new Date($this->entityManager, $this->languageManager, $this->stringTranslation);
+  }
+
+  /**
+   * Tests the formatPlugin method.
+   *
+   * @dataProvider providerTestFormatInterval
+   *
+   * @see \Drupal\Core\Datetime\Date::formatInterval()
+   */
+  public function testFormatInterval($interval, $granularity, $expected, $langcode = 'en') {
+    // Mocks a simple formatPlural implementation.
+    $this->stringTranslation->expects($this->any())
+      ->method('formatPlural')
+      ->with($this->anything(), $this->anything(), $this->anything(), array(), array('langcode' => $langcode))
+      ->will($this->returnCallback(function($count, $one, $multiple) {
+        return $count == 1 ? $one : str_replace('@count', $count, $multiple);
+      }));
+
+    $result = $this->date->formatInterval($interval, $granularity, $langcode);
+
+    $this->assertEquals($expected, $result);
+  }
+
+  /**
+   * Provides some test data for the format interval test.
+   */
+  public function providerTestFormatInterval() {
+    $data = array(
+      // Checks for basic seconds.
+      array(1, 1, '1 sec'),
+      array(1, 2, '1 sec'),
+      array(2, 1, '2 sec'),
+      array(2, 2, '2 sec'),
+      // Checks for minutes with seconds.
+      array(61, 1, '1 min'),
+      array(61, 2, '1 min 1 sec'),
+      array(62, 2, '1 min 2 sec'),
+      array(121, 1, '2 min'),
+      array(121, 2, '2 min 1 sec'),
+      // Check for hours with minutes and seconds.
+      array(3601, 1, '1 hour'),
+      array(3601, 2, '1 hour 1 sec'),
+      // Check for higher units.
+      array(86401, 1, '1 day'),
+      array(604800, 1, '1 week'),
+      array(2592000 * 2, 1, '2 months'),
+      array(31536000 * 2, 1, '2 years'),
+      // Check for a complicated one with months weeks and days.
+      array(2592000 * 2 + 604800 * 3 + 86400 * 4, 3, '2 months 3 weeks 4 days'),
+      // Check for the langcode.
+      array(61, 1, '1 min', 'xxx-lolspeak'),
+    );
+
+    return $data;
+  }
+
+  /**
+   * Tests the formatInterval method for 0 second.
+   */
+  public function testFormatIntervalZeroSecond() {
+    $this->stringTranslation->expects($this->once())
+      ->method('translate')
+      ->with('0 sec', array(), array('langcode' => 'xxx-lolspeak'))
+      ->will($this->returnValue('0 sec'));
+
+    $result = $this->date->formatInterval(0, 1, 'xxx-lolspeak');
+
+    $this->assertEquals('0 sec', $result);
+  }
+
+}