flexisip-account-manager/flexiapi/app/Providers/AppServiceProvider.php

<?php

namespace App\Providers;

use Illuminate\Support\Facades\Validator;
use Illuminate\Support\ServiceProvider;

class AppServiceProvider extends ServiceProvider
{
    public function register()
    {
        //
    }

    public function boot()
    {
        Validator::extend('iso_date', 'validateIsoDate');
    }
}