diff --git a/src/applications/calendar/import/PhabricatorCalendarImportEngine.php b/src/applications/calendar/import/PhabricatorCalendarImportEngine.php
--- a/src/applications/calendar/import/PhabricatorCalendarImportEngine.php
+++ b/src/applications/calendar/import/PhabricatorCalendarImportEngine.php
@@ -225,6 +225,7 @@
       $xactions[$full_uid] = $this->newUpdateTransactions($event, $node);
       $update_map[$full_uid] = $event;
 
+      $attendee_map[$full_uid] = array();
       $attendees = $node->getAttendees();
       $private_index = 1;
       foreach ($attendees as $attendee) {
@@ -526,7 +527,7 @@
       ->setStartDateTime($start_datetime)
       ->setEndDateTime($end_datetime);
 
-    $event->setIsAllDay($start_datetime->getIsAllDay());
+    $event->setIsAllDay((int)$start_datetime->getIsAllDay());
 
     // TODO: This should be transactional, but the transaction only accepts
     // simple frequency rules right now.
@@ -551,11 +552,18 @@
     PhabricatorUser $viewer,
     PhabricatorCalendarImport $import) {
 
-    $any_event = id(new PhabricatorCalendarEventQuery())
-      ->setViewer($viewer)
-      ->withImportSourcePHIDs(array($import->getPHID()))
-      ->setLimit(1)
-      ->execute();
+    $table = new PhabricatorCalendarEvent();
+    $conn = $table->establishConnection('r');
+
+    // Using a CalendarEventQuery here was failing oddly in a way that was
+    // difficult to reproduce locally (see T11808). Just check the table
+    // directly; this is significantly more efficient anyway.
+
+    $any_event = queryfx_all(
+      $conn,
+      'SELECT phid FROM %T WHERE importSourcePHID = %s LIMIT 1',
+      $table->getTableName(),
+      $import->getPHID());
 
     return (bool)$any_event;
   }
diff --git a/src/applications/calendar/notifications/PhabricatorCalendarNotificationEngine.php b/src/applications/calendar/notifications/PhabricatorCalendarNotificationEngine.php
--- a/src/applications/calendar/notifications/PhabricatorCalendarNotificationEngine.php
+++ b/src/applications/calendar/notifications/PhabricatorCalendarNotificationEngine.php
@@ -155,8 +155,9 @@
       $sent_map[$event_phid][$target_phid][$initial_epoch] = $row;
     }
 
-    $notify_min = $cursor;
-    $notify_max = $cursor + $this->getNotifyWindow();
+    $now = PhabricatorTime::getNow();
+    $notify_min = $now;
+    $notify_max = $now + $this->getNotifyWindow();
     $notify_map = array();
     foreach ($events as $key => $event) {
       $initial_epoch = $event->getUTCInitialEpoch();
diff --git a/src/applications/calendar/query/PhabricatorCalendarEventQuery.php b/src/applications/calendar/query/PhabricatorCalendarEventQuery.php
--- a/src/applications/calendar/query/PhabricatorCalendarEventQuery.php
+++ b/src/applications/calendar/query/PhabricatorCalendarEventQuery.php
@@ -126,7 +126,7 @@
     return array(
       'start' => array(
         'table' => $this->getPrimaryTableAlias(),
-        'column' => 'dateFrom',
+        'column' => 'utcInitialEpoch',
         'reverse' => true,
         'type' => 'int',
         'unique' => false,