Ensure the application uses the UTC timezone all over

We do not have the same access to PHP.ini through all development and
production environments. Consequently we set this in the application
kernel instead.

importers/config/services.yaml

@@ -5,6 +5,7 @@
 # https://symfony.com/doc/current/best_practices.html#use-parameters-for-application-configuration
 parameters:
     locale: 'en'
+    timezone: 'UTC'
 
 services:
     # default configuration for services in *this* file

importers/src/Kernel.php

@@ -8,4 +8,14 @@
 class Kernel extends BaseKernel
 {
     use MicroKernelTrait;
+
+    public function boot()
+    {
+        parent::boot();
+        $timezone = $this->getContainer()->getParameter('timezone');
+        if (!is_string($timezone)) {
+            throw new \InvalidArgumentException("Timezone parameter is not a string");
+        }
+        date_default_timezone_set($timezone);
+    }
 }