diff --git a/date_api/date_api.module b/date_api/date_api.module
index ca80f919f..1285b480f 100644
--- a/date_api/date_api.module
+++ b/date_api/date_api.module
@@ -1934,7 +1934,7 @@ function date_format_interval($date, $granularity = 2, $display_ago = TRUE) {
  *   A translated string representation of the interval.
  */
 function _date_format_interval($start_date, $end_date, $granularity = 2) {
-  $diff = $end_date->diff($start_date);
+  $diff = $start_date->diff($end_date);
 
   // As DateTime doesn't support weeks, calculate them and set the remaining
   // days.
diff --git a/date_api/tests/DateApiTestCase.test b/date_api/tests/DateApiTestCase.test
index 7f6bb5b65..789d96aae 100644
--- a/date_api/tests/DateApiTestCase.test
+++ b/date_api/tests/DateApiTestCase.test
@@ -464,6 +464,82 @@ class DateApiTestCase extends DrupalWebTestCase {
     $this->assertEqual('1980-01-01', $obj->format('Y-m-d'));
   }
 
+  /**
+   * Test the date_format_interval() function.
+   */
+  public function testDateInterval() {
+    // Return values expected from date_format_interval(), keyed by input to
+    // strtotime().
+    $expected_intervals = array(
+      // Any interval that contains fractions of a day (e.g. 1 hour) will fail
+      // if the current time is that interval before midnight (e.g. 23:00:00).
+      // This is due to the constructor of DateObject considering any date with
+      // a time of 00:00:00 as "dateOnly", resulting in a "1 day" interval
+      // instead of "1 hour" as the times are ignored.
+      '+ 5 min' => '5 min',
+      '+ 1 hour' => '1 hour',
+      '+ 2 hours' => '2 hours',
+      '+ 1 day' => '1 day',
+      '+ 1 week' => '1 week',
+      '+ 1 week 3 days' => '1 week 3 days',
+      '+ 1 week 5 days' => '1 week 5 days',
+      '+ 1 week 6 days' => '1 week 6 days',
+      '+ 2 weeks' => '2 weeks',
+      '+ 2 weeks 1 day' => '2 weeks 1 day',
+      '+ 2 weeks 2 days' => '2 weeks 2 days',
+      '+ 1 month' => '1 month',
+      '+ 1 month 1 week 1 day' => '1 month 1 week 1 day',
+      '+ 1 month 1 week 2 days' => '1 month 1 week 2 days',
+      '+ 1 month 1 week 6 days' => '1 month 1 week 6 days',
+      '+ 1 month 2 weeks 1 day' => '1 month 2 weeks 1 day',
+      '+ 1 month 2 weeks 2 days' => '1 month 2 weeks 2 days',
+      '+ 1 month 3 weeks 6 days' => '1 month 3 weeks 6 days',
+      '+ 2 months' => '2 months',
+      '+ 3 months' => '3 months',
+      '+ 11 months' => '11 months',
+      '+ 12 months' => '1 year',
+      '+ 3 months 3 days' => '3 months 3 days',
+      '+ 1 year' => '1 year',
+      '+ 1 year 1 month 1 week 1 day' => '1 year 1 month 1 week 1 day',
+      '+ 1 year 2 months 3 weeks 4 days 5 hours 6 min 7 sec' => '1 year 2 months 3 weeks 4 days 5 hours 6 min 7 sec',
+      '+ 3 years 3 months 3 weeks 3 days 3 hours 3 min 3 sec' => '3 years 3 months 3 weeks 3 days 3 hours 3 min 3 sec',
+      '+ 100 years' => '100 years',
+      '- 1 hour' => '1 hour ago',
+      '- 1 day' => '1 day ago',
+      '- 1 week' => '1 week ago',
+      '- 2 weeks' => '2 weeks ago',
+      '- 1 month' => '1 month ago',
+      '- 2 months' => '2 months ago',
+      '- 1 year' => '1 year ago',
+      '- 100 years' => '100 years ago',
+    );
+
+    // Change timezones used by PHP and Drupal functions to UTC so we don't have
+    // to deal with daylight savings time offsets that occur if intervals
+    // include a DST change.
+    $old_timezone_php = date_default_timezone_get();
+    $old_timezone_drupal = variable_get('date_default_timezone');
+    variable_set('date_default_timezone', 'UTC');
+    date_default_timezone_set('UTC');
+
+    foreach ($expected_intervals as $input => $expected) {
+      // Use REQUEST_TIME like date_format_interval() as time() could be some
+      // seconds off.
+      $end_date = new DateObject($timestamp = strtotime($input, REQUEST_TIME));
+      $formatted = date_format_interval($end_date, 7);
+
+      // Start and end of interval formatted for output.
+      $now = (new DateObject(REQUEST_TIME))->format(DateTimeInterface::ATOM);
+      $end = $end_date->format(DateTimeInterface::ATOM);
+
+      $this->assertEqual($expected, $formatted, "Test formatted interval now/$now $input (= $end): should be '$expected', found '$formatted'.");
+    }
+
+    // Reset timezones to previous values.
+    date_default_timezone_set($old_timezone_php);
+    variable_set('date_default_timezone', $old_timezone_drupal);
+  }
+
   /**
    * Tear down tests.
    */