config: Add [system] => 'timezone'

RSS-Bridge currently statically sets the timezone to UTC which can
result in incorrect timestamps if the server is hosted in another
region.

This commit adds a new configuration parameter to allow admins to
specify their own timezone for their servers. Invalid values will
result in an error message.

Example:

  [system]
  timezone = "UTC"

For compatibility reasons the default value is set to UTC.

This parameter accepts any of the supported timezones listed at
https://www.php.net/manual/en/timezones.php

Closes #956
References #1001

config.default.ini.php

@@ -4,6 +4,14 @@
 ; file, it will be replaced on the next update of RSS-Bridge! You can specify
 ; your own configuration in 'config.ini.php' (copy this file).
 
+[system]
+
+; Defines the timezone used by RSS-Bridge
+; Find a list of supported timezones at
+; https://www.php.net/manual/en/timezones.php
+; timezone = "UTC" (default)
+timezone = "UTC"
+
 [cache]
 
 ; Defines the cache type used by RSS-Bridge

lib/Configuration.php

@@ -155,6 +155,12 @@ public static function loadConfiguration() {
 			}
 		}
 
+		if(!is_string(self::getConfig('system', 'timezone'))
+		|| !in_array(self::getConfig('system', 'timezone'), timezone_identifiers_list(DateTimeZone::ALL_WITH_BC)))
+			die('Parameter [system] => "timezone" is invalid! Please check "config.ini.php"!');
+
+		date_default_timezone_set(self::getConfig('system', 'timezone'));
+
 		if(!is_string(self::getConfig('proxy', 'url')))
 			die('Parameter [proxy] => "url" is not a valid string! Please check "config.ini.php"!');