diff --git a/lib/contents.php b/lib/contents.php
index a3830ca7..055d6bf3 100644
--- a/lib/contents.php
+++ b/lib/contents.php
@@ -63,8 +63,13 @@ function getContents(
     if ($cachedResponse) {
         $cachedLastModified = $cachedResponse->getHeader('last-modified');
         if ($cachedLastModified) {
-            $cachedLastModified = new \DateTimeImmutable($cachedLastModified);
-            $config['if_not_modified_since'] = $cachedLastModified->getTimestamp();
+            try {
+                // Some servers send Unix timestamp instead of RFC7231 date. Prepend it with @ to allow parsing as DateTime
+                $cachedLastModified = new \DateTimeImmutable((is_numeric($cachedLastModified) ? '@' : '') . $cachedLastModified);
+                $config['if_not_modified_since'] = $cachedLastModified->getTimestamp();
+            } catch (Exception $dateTimeParseFailue) {
+                // Ignore invalid 'Last-Modified' HTTP header value
+            }
         }
     }