how to make a date based blog archieve in ci3 using query builder

Try this:

Code:

$this->db->select('DATE_FORMAT(datetime,"%Y%m") AS month',NULL,FALSE)
->select('COUNT(id) AS num_per_month')
->from('posts')
->group_by('month');
$query = $this->db->get()->result_array();

Assuming that your table name is 'posts' and this table has a field called 'datetime'.
The result is an array with rows. Each row has the columns 'month' and 'num_per_month'.
E.g.: $r['month'] => '201501' stands for: January 2015.