The default timezone for PHP is UTC regardless of your server's timezone. This is the timezone used by all PHP date/time functions in your scripts.
To change the PHP timezone for an app, create a .user.ini file in the app's public directory with the following contents:
date.timezone = America/Los_Angeles
See PHP's list of supported timezones to find the names of all possible timezones you can use for the date.timezone setting.
For information on changing other PHP settings, see our tutorial on using .user.ini files.