My solution is: date('Y-m-d', time()-60*60*24*365);
You can make it more "readable" with defines:
define('ONE_SECOND', 1);
define('ONE_MINUTE', 60 * ONE_SECOND);
define('ONE_HOUR', 60 * ONE_MINUTE);
define('ONE_DAY', 24 * ONE_HOUR);
define('ONE_YEAR', 365 * ONE_DAY);
date('Y-m-d', time()-ONE_YEAR);