How to insert NOW() in DATETIME field

I'm building a seeder to populate the database, but I'm not able to set NOW() in DATETIME fields. How can I do it?

I tried this:

PHP Code:

    public function run() {
        helper('date');
        $now = now();
        
        $data = [
            [
                'user_id' => 1,
                'group_id' => 1,
                'created_at' => $now,
                'updated_at' => $now
            ],
            [
                'user_id' => 1,
                'group_id' => 2,
                'created_at' => $now,
                'updated_at' => $now
            ],
            [
                'user_id' => 2,
                'group_id' => 2,
                'created_at' => $now,
                'updated_at' => $now
            ]
        ];

        $this->db->table('back_user_groups_rel')->insertBatch($data);
    } 


And also with literal 'NOW()' here:

PHP Code:

$now = 'NOW()'; 


None of them works.