diff --git a/src/JsonSchema/Constraints/FormatConstraint.php b/src/JsonSchema/Constraints/FormatConstraint.php
index 12c6a2bf..42dec55d 100644
--- a/src/JsonSchema/Constraints/FormatConstraint.php
+++ b/src/JsonSchema/Constraints/FormatConstraint.php
@@ -127,7 +127,19 @@ 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', $format) !== -1) && (preg_match('/\.\d+Z$/', $datetime))) {
+            return true;
+        }
+
+        return false;
     }
 
     protected function validateRegex($regex)
diff --git a/tests/Constraints/FormatTest.php b/tests/Constraints/FormatTest.php
index eb47983d..f604c083 100644
--- a/tests/Constraints/FormatTest.php
+++ b/tests/Constraints/FormatTest.php
@@ -81,6 +81,10 @@ public function getValidFormats()
             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('2000-05-01T12:12:12.123000Z', 'date-time'),
+            array('2000-05-01T12:12:12.0Z', 'date-time'),
+            array('2000-05-01T12:12:12.000Z', 'date-time'),
+            array('2000-05-01T12:12:12.000000Z', 'date-time'),
 
             array('0', 'utc-millisec'),
 
@@ -140,6 +144,7 @@ public function getInvalidFormats()
             array('1999-1-11T00:00:00Z', 'date-time'),
             array('1999-01-11T00:00:00+100', 'date-time'),
             array('1999-01-11T00:00:00+1:00', 'date-time'),
+            array('1999.000Z-01-11T00:00:00+1:00', 'date-time'),
 
             array('-1', 'utc-millisec'),
             array(PHP_INT_MAX, 'utc-millisec'),