A sample implementation of redefining a public static method of symfony's sfDate to 'travel in time' with runkit:
<?php
//Going back in time 3 days
$subtractablePeriod = 3;
runkit_method_redefine('sfDateTimeToolkit', 'now', '', 'return strtotime("-'.$subtractablePeriod.' days");', (RUNKIT_ACC_PUBLIC | RUNKIT_ACC_STATIC));
//Do Stuff here that would otherwise get the current date
//Return to realtime
runkit_method_redefine('sfDateTimeToolkit', 'now', '', 'return time();', (RUNKIT_ACC_PUBLIC | RUNKIT_ACC_STATIC));
?>