为什么PHP认为我的日期是一个字符串？

My code is giving me the error "Fatal error: Call to a member function sub() on string" As you can see the $date variable is a string based on the echo, but it still believes it to be a string.

My code:

echo $stringDate;
    //PRINTS "2014-03-27"
$date = date('yyyy-mm-dd', strtotime( $stringDate );
echo $date;
    //PRINTS "14141414-0303-2727"
$date->sub(new date_interval_create_from_date_string('70 days') );
    //Produces "Fatal error: Call to a member function sub() on string"
$milestones["Method Development Checklist"]['date'] = $date;

Instead of

date('yyyy-mm-dd', strtotime( $stringDate ));

use

date_create_from_format("Y-m-d", $stringDate);

which is an alias for

DateTime::createFromFormat("Y-m-d", $stringDate);

Have a nice day o/