Two very useful functions in PHP is strtotime and date. It can be cumbersome to make use of these 2 though, so I got a little function that makes it so much easier.

function odate($format, $criteria){
  return (date($format), strtotime($criteria, strtotime($format)));
}

Now all you do is call it like :

echo odate("Y-m-d H:i:s", "+1 days"); // 1 day ahead of today
echo odate("Y-m-d H:i:s", "-7 days"); //  7 days ago
echo odate("Y-m-d H:i:s", "+1 hours"); // 1 hours from now