Timezone Keeper is a service for get default/system timezone and get user timezone and date from it.
As a default used ResolveAndKeep timezone keeper, but you can override it and implement KeeperInterface. Default
timezone keeper use CollectionResolver service for get all available timezone resolvers and use it for get user
timezone. See guide how to create custom timezone resolver.
You can get timezone keeper from DI:
$tz_keeper = $this->get('gpslab.date.tz.keeper');