my database table looks like below
| id | user_name | address | contact | date |
|----|-----------|---------|---------|----------|
| 1 | john | NY
You can use the following query to get last 15 days data based on your localhost time zone as may be your MYSQL database time zone is different than your localhost then you will not get correct data from database.
$result = $this->db->query('SELECT * FROM '.$table.' WHERE date >= '.date('Y-m-d', time() - (15 * 24 * 60 * 60)).' AND date <= '.date('Y-m-d').' AND '.$condition);