As previously mentioned, to do a negative DateInterval object, you'd code:
<?php
$date1 = new DateTime();
$eightynine_days_ago = new DateInterval( "P89D" );
$eightynine_days_ago->invert = 1; //Make it negative.
$date1->add( $eightynine_days_ago );
?>
and then $date1 is now 89 days in the past.