I thought this was pretty neat and useful. You can combine strtotime with the date function to create your own date verbally. This is just an alternative to using mktime that's a little more intuitive.
<?php
$date = date('Y-m-d', strtotime("-30 days"));
$date = date('Y-m-d', strtotime("-10 years"));
$date = date('Y-m-d', strtotime("+30 days"));
?>