diff --git a/src/Monolog/Formatter/LogglyFormatter.php b/src/Monolog/Formatter/LogglyFormatter.php
index 5ef0d9fa..f02bceb0 100644
--- a/src/Monolog/Formatter/LogglyFormatter.php
+++ b/src/Monolog/Formatter/LogglyFormatter.php
@@ -38,7 +38,7 @@ class LogglyFormatter extends JsonFormatter
     public function format(array $record)
     {
         if (isset($record["datetime"]) && ($record["datetime"] instanceof \DateTime)) {
-            $record["timestamp"] = $record["datetime"]->format("c");
+            $record["timestamp"] = $record["datetime"]->format("Y-m-d\TH:i:s.uO");
             // TODO 2.0 unset the 'datetime' parameter, retained for BC
         }