diff --git a/resources/sql/autopatches/20170410.calendar.01.repair.php b/resources/sql/autopatches/20170410.calendar.01.repair.php
new file mode 100644
--- /dev/null
+++ b/resources/sql/autopatches/20170410.calendar.01.repair.php
@@ -0,0 +1,42 @@
+<?php
+
+// See T12488. Some events survived "20161004.cal.01.noepoch.php" without
+// having "utcInstanceEpoch" computed, which breaks ICS export. This appears
+// to be the result of some bug which has been fixed in the meantime, so just
+// redo this part of the migration.
+
+$table = new PhabricatorCalendarEvent();
+$conn = $table->establishConnection('w');
+$table_name = $table->getTableName();
+
+$viewer = PhabricatorUser::getOmnipotentUser();
+$all_events = id(new PhabricatorCalendarEventQuery())
+  ->setViewer($viewer)
+  ->execute();
+foreach ($all_events as $event) {
+  $id = $event->getID();
+
+  if (!$event->getInstanceOfEventPHID()) {
+    // Not a child event, so no instance epoch.
+    continue;
+  }
+
+  if ($event->getUTCInstanceEpoch()) {
+    // Already has an instance epoch.
+    continue;
+  }
+
+  try {
+    $event->updateUTCEpochs();
+  } catch (Exception $ex) {
+    phlog($ex);
+    continue;
+  }
+
+  queryfx(
+    $conn,
+    'UPDATE %T SET utcInstanceEpoch = %nd WHERE id = %d',
+    $table_name,
+    $event->getUTCInstanceEpoch(),
+    $id);
+}