When you want to set the default timezone used by all date/time functions, you can take one of two options: 1) put date_default_timezone_set() on your code, 2) put value in php.ini

Option 1 - by code

You can set the default timezone by following function in the PHP

date_default_timezone_set ( string $timezone_identifier ) : bool

And the below is the example:

date_default_timezone_set('Asia/Seoul');

You can get more timezone information at https://www.php.net/manual/en/timezones.php

Option 2 - by php.ini

Below is simple, and no need to add code every time, but may need to be careful when you have any possibility to migrate onto the different systems that does not have the same php.ini settings.

[Date]
; Defines the default timezone used by the date functions
; http://php.net/date.timezone
date.timezone = Asia/Seoul