diff --git a/src/Authentication/Timestamp.php b/src/Authentication/Timestamp.php
index bbc87f4..923ce3d 100644
--- a/src/Authentication/Timestamp.php
+++ b/src/Authentication/Timestamp.php
@@ -18,6 +18,8 @@
  */
 class Timestamp
 {
+    const FORMAT = 'Ymd\TH:i:sO';
+
     /**
      * @var \DateTime Signing Timestamp
      */
@@ -69,6 +71,6 @@ public function setValidFor($interval)
      */
     public function __toString()
     {
-        return $this->timestamp->format('Ymd\TH:i:s.uO');
+        return $this->timestamp->format(static::FORMAT);
     }
 }
diff --git a/tests/Authentication/TimestampTest.php b/tests/Authentication/TimestampTest.php
index 853b96f..93f8a7c 100644
--- a/tests/Authentication/TimestampTest.php
+++ b/tests/Authentication/TimestampTest.php
@@ -28,8 +28,8 @@ public function testTimestampFormat()
     {
         $timestamp = new \Akamai\Open\EdgeGrid\Authentication\Timestamp();
 
-        $check = \DateTime::createFromFormat('Ymd\TH:i:s.uO', (string) $timestamp, new \DateTimeZone('UTC'));
-        $this->assertEquals($check, \PHPUnit_Framework_Assert::readAttribute($timestamp, 'timestamp'));
+        $check = \DateTime::createFromFormat('Ymd\TH:i:sO', (string) $timestamp, new \DateTimeZone('UTC'));
+        $this->assertEquals($check->format(\Akamai\Open\EdgeGrid\Authentication\Timestamp::FORMAT), (string) $timestamp);
     }