Revert back change

Author: bnymn

app/code/Magento/Newsletter/Model/Queue.php

@@ -196,8 +196,7 @@ public function setQueueStartAtByString($startAt)
         if ($startAt === null || $startAt == '') {
             $this->setQueueStartAt(null);
         } else {
-            $startAt = $this->timezone->convertConfigTimeToUtcWithPattern($startAt, 'Y-m-d H:i:s', null);
-            $this->setQueueStartAt($startAt);
+            $this->setQueueStartAt($this->timezone->convertConfigTimeToUtc($startAt));
         }
         return $this;
     }