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
@@ -526,7 +526,7 @@
       ->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 +551,18 @@
     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/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,