diff --git a/test/server/SimpleServer.js b/test/server/SimpleServer.js
index 88595761..4e7db5a0 100644
--- a/test/server/SimpleServer.js
+++ b/test/server/SimpleServer.js
@@ -221,7 +221,7 @@ class SimpleServer {
         return;
       }
       response.setHeader('Cache-Control', 'public, max-age=31536000');
-      response.setHeader('Last-Modified', this._startTime.toString());
+      response.setHeader('Last-Modified', this._startTime.toISOString());
     } else {
       response.setHeader('Cache-Control', 'no-cache, no-store');
     }