diff --git a/Tests/Transformer/ModelToElasticaAutoTransformerTest.php b/Tests/Transformer/ModelToElasticaAutoTransformerTest.php
index 6f2aaea..fe979e7 100644
--- a/Tests/Transformer/ModelToElasticaAutoTransformerTest.php
+++ b/Tests/Transformer/ModelToElasticaAutoTransformerTest.php
@@ -149,7 +149,7 @@ class ModelToElasticaAutoTransformerTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals(true, $data['bool']);
         $this->assertEquals(false, $data['falseBool']);
         $expectedDate = new \DateTime('1979-05-05');
-        $this->assertEquals($expectedDate->format('U'), $data['date']);
+        $this->assertEquals($expectedDate->format('c'), $data['date']);
     }
 
     public function testThatCanTransformObjectWithIteratorValue()
@@ -186,7 +186,7 @@ class ModelToElasticaAutoTransformerTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals(
             array(
                  'key1'  => 'value1',
-                 'key2'  => array('value2', false, 123, 8.9, $expectedDate->format('U')),
+                 'key2'  => array('value2', false, 123, 8.9, $expectedDate->format('c')),
             ), $data['multiArray']
         );
     }
diff --git a/Transformer/ModelToElasticaAutoTransformer.php b/Transformer/ModelToElasticaAutoTransformer.php
index 1666beb..ab0f339 100644
--- a/Transformer/ModelToElasticaAutoTransformer.php
+++ b/Transformer/ModelToElasticaAutoTransformer.php
@@ -107,7 +107,7 @@ class ModelToElasticaAutoTransformer implements ModelToElasticaTransformerInterf
         $normalizeValue = function(&$v)
         {
             if ($v instanceof \DateTime) {
-                $v = $v->format('v');
+                $v = $v->format('c');
             } elseif (!is_scalar($v) && !is_null($v)) {
                 $v = (string)$v;
             }