Change Q2A timezone in timestamp

Question

I can't change the timezone to GMT +7, I have tried:
- Edit qa-base file (line 137)

if (function_exists('date_default_timezone_set') && function_exists('date_default_timezone_get'))

@date_default_timezone_set("Asia/Ho_Chi_Minh"); 

- Try to set default timezone in qa-config.

- Check the date stored in database, and it's stored in GMT +7. So the problem is the PHP timezone.

So anyone have experience this? Please help me, thanks a lot.