diff -u b/src/Plugin/DateAugmenter/AddToCal.php b/src/Plugin/DateAugmenter/AddToCal.php
--- b/src/Plugin/DateAugmenter/AddToCal.php
+++ b/src/Plugin/DateAugmenter/AddToCal.php
@@ -207,8 +207,9 @@
     $google_link['text'] = $label;
 
     // Dates.
-    $now->setTimezone(new \DateTimeZone($timezone));
-    $ical_link[] = 'DTSTAMP:' . $now->format('Ymd\\THi00');
+    // As per RFC 2445 4.8.7.2 the DTSTAMP property must be in UTC.
+    $now->setTimezone(new \DateTimeZone('UTC'));
+    $ical_link[] = 'DTSTAMP:' . $now->format('Ymd\\THi00\\Z');
     $ical_link[] = 'DTSTART' . $prefix . $start_formatted;
     $ical_link[] = 'DTEND' . $prefix . $end_formatted;
     $google_link['dates'] = $start_formatted . '/' . $end_formatted;