Use the factory to get instances of EDTFInfo:
$factory = new \ComputerMinds\EDTF\EDTFInfoFactory();
$dateInfo = $factory->create('1990-01');
Then you can call the various methods on the instance.
$valid = $dateInfo->isValid();
if ($valid) {
$min = $dateInfo->getMin();
$max = $dateInfo->getMax();
// $min and $max are just standard PHP \DateTime instances.
print $min->format('c');
}