diff --git a/src/JsonSchema/Constraints/FormatConstraint.php b/src/JsonSchema/Constraints/FormatConstraint.php
index c00baad6..c7897538 100644
--- a/src/JsonSchema/Constraints/FormatConstraint.php
+++ b/src/JsonSchema/Constraints/FormatConstraint.php
@@ -130,7 +130,19 @@ protected function validateDateTime($datetime, $format)
             return false;
         }
 
-        return $datetime === $dt->format($format);
+        if ($datetime === $dt->format($format)) {
+            return true;
+        }
+
+        // handles the case where a non-6 digit microsecond datetime is passed
+        // which will fail the above string comparison because the passed
+        // $datetime may be '2000-05-01T12:12:12.123Z' but format() will return
+        // '2000-05-01T12:12:12.123000Z'
+        if ((strpos('u', $format) !== -1) && (intval($dt->format('u')) > 0)) {
+            return true;
+        }
+
+        return false;
     }
 
     protected function validateRegex($regex)
diff --git a/tests/JsonSchema/Tests/Constraints/FormatTest.php b/tests/JsonSchema/Tests/Constraints/FormatTest.php
index 30797237..eb47983d 100644
--- a/tests/JsonSchema/Tests/Constraints/FormatTest.php
+++ b/tests/JsonSchema/Tests/Constraints/FormatTest.php
@@ -17,7 +17,7 @@ public function setUp()
     {
         date_default_timezone_set('UTC');
     }
-    
+
     public function testNullThing()
     {
         $validator = new FormatConstraint();
@@ -80,6 +80,7 @@ public function getValidFormats()
             array('2000-05-01T12:12:12+0100', 'date-time'),
             array('2000-05-01T12:12:12+01:00', 'date-time'),
             array('2000-05-01T12:12:12.123456Z', 'date-time'),
+            array('2000-05-01T12:12:12.123Z', 'date-time'),
 
             array('0', 'utc-millisec'),