Can any one please help where we can do set default timezone in vanilla
Your application should have a file called /applications/YourApplication/settings/class.yourapplicationhooks.php which has a setup() method (use the hooks file of the vanilla application as an example).
In this method you should add the following line: touchConfig('Garden.GuestTimeZone', 'Europe/Berlin');
It will set the default time zone for guests but only when it has not been set before. There is no need for a default time zone for users since their timezone gets determined when they create an account/log in.
Ok Thanks @R_J