$now = new DateTime();
$zero = new DateTime('0000-00-00'); // -0001-11-30 - Nov 30, 1 BC. Interesting.
$diff = $now->diff($zero);
echo $diff->format('%a days'); // 735728 days

$now = new DateTime();
$zero = new DateTime('0001-01-01');
$diff = $now->diff($zero);
echo $diff->format('%a days'); // 735330 days

echo (new DateTime())->diff(new DateTime('0001-01-01'))->format('%a days');

function toDays($date) {
    return (new DateTime())->diff(new DateTime($date))->format('%a');
}

$days  = date_create('now')->diff(date_create('0000-00-00'))->format('%a');
$years = floor($days / 365.242); // => 2014