diff --git a/src/Composer/Package/Locker.php b/src/Composer/Package/Locker.php
index b11f5a975..93e5ca655 100644
--- a/src/Composer/Package/Locker.php
+++ b/src/Composer/Package/Locker.php
@@ -432,6 +432,6 @@ class Locker
             }
         }
 
-        return $datetime ? $datetime->format('Y-m-d H:i:s') : null;
+        return $datetime ? $datetime->format(DATE_RFC3339) : null;
     }
 }