Registering and running own debug function in Symfony

I am developing a web app using Symfony 5.4 and my current goal is to define and run a function

function setTestDate(): void
{
    testdate = $_REQUEST['testdate'] ?? null;
    if ((int)$_SERVER['APP_DEBUG'] && $testdate !== null) {
        CarbonCarbon::setTestNow($testdate);  //  provided by Carbon, a real life-saver
    }
}

as early as possible, already before the Authenticators are called. I previously tried to run this during the event ‘kernel.request’ but this was already too late in the event flow: My authenticator checks the age of a certain token and without a fixed testdate my fixtures would have to be updated all the time.

I know of many bad places to insert this into (index.php, the Authenticator itself, etc), but does anybody know of a “clean” way to call that function, maybe during SymfonyComponentErrorHandlerDebug::enable()? It seems like none of the built-in events are that early and I’d hate to edit index.php. I am aware of onVoterVote(), but this testdate should be used even if no Authenticator was called.

I guess this looks like I’m over-engineering a minor problem, but I am continuously trying to improve my coding style.