changed laravel app timezone, time got from db did not change - laravel

Question

Assume having a Eloquent model called Request with the column next_check autocasted as date (protected $dates = ['next_check'];).
APP_TIMEZONE is initial Europe/Helsinki (+02:00)
$ artisan tinker
Psy Shell v0.10.4 (PHP 7.4.11 — cli) by Justin Hileman
>>> $pr = Request::find(1);
>>> $pr->next_check = Carbon\Carbon::now();
>>> $pr->save();
>>> Request::find(1)->next_check;
=> Illuminate\Support\Carbon #1603973824 {#2900
date: 2020-10-29 14:17:04.0 Europe/Helsinki (+02:00),
}
That looks all fine and we saved 14:17:04 assuming we're in Helsinki.
Now changing the timezone to Europe/Berlin (+01:00), I would assume to get 13:17:04 returned.
>>> Request::find(1)->next_check;
=> Illuminate\Support\Carbon #1603977424 {#2900
date: 2020-10-29 14:17:04.0 Europe/Berlin (+01:00),
}
The timezone is changed but still there is the time returned, which was originally saved in Europe/Helsinki time.
Is that an expected behaviour and how can I get my desired behaviour that a change of the app timezone would result in converted times?
p.s.: the db timezone configured in laravel is still the default '+00:00'

Answer 1

If you want to localise times so they show to user in the user's timezone, you need to CONVERT an existing datetime:
$users_timezone = 'Europe/Berlin';
$datetime->setTimezone($users_timezone);
This will actually change the time according to the difference between the app's timezone and the one converted to.

Answer 2

To get my expected behaviour, I now found a solution. If anybody else want's that exact same behaviour, this might be the solution:
Add the following call in your AppServiceProvider's boot-method:
$this->setUTCOffsetInDBConnection();
The setUTCOffsetInDBConnection method's code is as following:
// according to https://www.sitepoint.com/synchronize-php-mysql-timezone-configuration/
private function setUTCOffsetInDBConnection() {
$now = new \DateTime();
$mins = $now->getOffset() / 60;
$sgn = ($mins < 0 ? -1 : 1);
$mins = abs($mins);
$hrs = floor($mins / 60);
$mins -= $hrs * 60;
$offset = sprintf('%+d:%02d', $hrs*$sgn, $mins);
DB::statement("SET time_zone='".$offset."';");
}
This will always synchronize the timezone according to your app's timezone and save the timestamps as UTC in the database.
When you now save a timestamp 2020-10-29 15:28:00 from your app with APP_TIMEZONE=Europe/Berlin in your .env, it will save it as UTC in the database (2020-10-29 15:28:00).
When you change your APP_TIMEZONE to Europe/Helsinki now, you will get 2020-10-29 16:28:00 returned.
Be aware that your phpMyAdmin might still not show the saved UTC times in your database since also phpMyAdmin sets the servers timezone when opening the connection. So if your server which runs phpMyAdmin is also in Europe/Berlin, you will see 2020-10-29 15:28:00 and not the UTC date saved in the DB.

Answer 3

Answer to "Is that an expected behaviour" is YES
To "save" the timezone with the date data, you have to save it too as an extra column.
So having both "2020-10-29 14:17:04.0" and "Europe/Helsinki" in DB will allow you to remember what timezone was used to save this date, so if it's changed, you can convert.
But first you should consider not to change your app and DB timezone, both should remain UTC.
See https://medium.com/#kylekatarnls/always-use-utc-dates-and-times-8a8200ca3164
So so you can easily convert them into any timezone and display it properly according to the user timezone while keeping a standard well-known format in DB.