diff --git a/src/Encoder/PaymentRequestDecoder.php b/src/Encoder/PaymentRequestDecoder.php
index 757b443..4ce4844 100644
--- a/src/Encoder/PaymentRequestDecoder.php
+++ b/src/Encoder/PaymentRequestDecoder.php
@@ -167,9 +167,13 @@ public function decode(string $paymentRequest): array
         $timeExpireDate = $timeExpireDateString = null;
         if ($this->tagsContainItem($tags, $this->tagNames[6])) {
             $timeExpireDate = $timestamp + $this->tagsItems($tags, $this->tagNames[6]);
-            $timeExpireDateString = date(\DateTime::ATOM, $timeExpireDate);
+        } else {
+            // Default is 3600 (1 hour) if not specified.
+            $timeExpireDate = $timestamp + 3600;
         }
 
+        $timeExpireDateString = date(\DateTime::ATOM, $timeExpireDate);
+
         $toSign = Buffertools::concat(
             new Buffer($prefix),
             $this->wordsToBuffer($wordsWithoutSignature, false),