Add PHP function to Volt

Hello, i want to use PHP's strtotime function on volt. I followed the tutorial and include the following code in the services.php file:

$view = new View();
$volt = new Volt($view, $di);
$compiler = $volt->getCompiler();
$compiler->addFunction('strtotime', 'strtotime');

But, when i tried it, it says strtotime is undefined. Is there a specific place where i need to put those code or there's something else ?