mysql - less than or greater than in where clause in codeigniter model -

i want execute query

update `eventinfo` set `status` = '0' `eventdatetime`< `2015-05-12 01:17:23`. 

i tried in different ways this

$whereclause = 'eventdatetime'.'<'.$check_date; $this->db->where($whereclause); 

but failed.what correct way.

different way :

$data = array( 'status' => '0',  );  $this->db->where('eventdatetime <', '2015-05-12 01:17:23'); $this->db->update('eventinfo', $data);