Database Timezone

Hi to all developers...

In my web app, some of datetime insert automatically on insert data (using timestamp) and some manually (set with php date function and then insert). I've set date_default_timezone_set in index.php and everything is well on local. Now I move app on live server and on testing, see the timestamp (on insert a row in db) is different from date_default_timezone_set!!!

I search a lot and see other's guide, but nothing happened!
"$this->db->query('SET time_zone = ""');",  
"add date_default_timezone_set", 
"change $config['time_reference'] in config file", 
"create a parent model and set time zone in that" or... 
none of them did not worked for me!

How could solve this? Could you guide me to handle this issue?

thanks