If I am not misunderstand, you are trying fetch current datetime from database, but it generates an instance of Express, not a current datetime value, you should execute your query:
$expression = new Expression('NOW()');
$now = (new \yii\db\Query)->select($expression)->scalar(); // SELECT NOW();
$this->field_date = $now;
But I think it is not the right way to do that!
Firstly, it is no need to retrieve current time from database, use PHP instead to avoid increasing database overhead.
Secondly, the time could be affected by the database’s time zone, it will causes some problems if the PHP zone time is different with database time zone.
It is recommended that always use PHP to retrieve datetime, we just need to concern about PHP zone time:
$this->field_date = date('Y-m-d H:i:s');