diff --git a/src/DirectMailTransport.php b/src/DirectMailTransport.php
index d521ef8..5ade8bf 100644
--- a/src/DirectMailTransport.php
+++ b/src/DirectMailTransport.php
@@ -36,7 +36,7 @@ public function __construct($AccessKeyId, $AccessSecret, $ReplyToAddress, $Addre
         $this->CommonParameters['Version'] = '2015-11-23';
         $this->CommonParameters['AccessKeyId'] = $AccessKeyId;
         $this->CommonParameters['SignatureMethod'] = 'HMAC-SHA1';
-        $this->CommonParameters['Timestamp'] = date('Y-m-d\TH:i:s\Z');
+        $this->CommonParameters['Timestamp'] = gmdate('Y-m-d\TH:i:s\Z');
         $this->CommonParameters['SignatureVersion'] = '1.0';
         $this->CommonParameters['SignatureNonce'] = uniqid();
     }