Prevent inserting 0 year into db - time difference

I read an article about time difference in stackoverflow URL

$datetime1 = new DateTime("2010-06-20");

$datetime2 = new DateTime("2011-06-22");

$difference = $datetime1->diff($datetime2);

echo 'Difference: '.$difference->y.' years, ' 
                   .$difference->m.' months, ' 
                   .$difference->d.' days';

print_r($difference);

This is a code to calculate time difference as you know , and i need a time diff to insert into db , but if the time diff is less than 0 years it inserts the 0 year too , how can i prevent this?