diff --git a/src/JsonSchema/Constraints/FormatConstraint.php b/src/JsonSchema/Constraints/FormatConstraint.php
index c7897538..85a33e28 100644
--- a/src/JsonSchema/Constraints/FormatConstraint.php
+++ b/src/JsonSchema/Constraints/FormatConstraint.php
@@ -138,7 +138,7 @@ protected function validateDateTime($datetime, $format)
         // 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)) {
+        if ((strpos('u', $format) !== -1) && (preg_match('/\.\d+Z/', $datetime))) {
             return true;
         }
 
diff --git a/tests/JsonSchema/Tests/Constraints/FormatTest.php b/tests/JsonSchema/Tests/Constraints/FormatTest.php
index eb47983d..f604c083 100644
--- a/tests/JsonSchema/Tests/Constraints/FormatTest.php
+++ b/tests/JsonSchema/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'),