PHP
(quick examples)
ORIGINAL TIME $dt = new DateTime("2019-08-07");
ADD DATE $dt->modify("+4 days"); echo $dt->format("d M Y");
SUBTRACT DATE $dt->modify("-2 weeks"); echo $dt->format("d M Y");
ORIGINAL TIME $date = "2019-08-07";
ADD DATE echo date("d M Y", strtotime($date."+2 days"));
SUBTRACT DATE echo date("d M Y", strtotime($date."-3 weeks"));
UNIX TIMESTAMP $unix = strtotime("2019-08-07");
ADD DATE echo date("d M Y", $unix + (4*$day));
SUBTRACT DATE echo date("d M Y", $unix - (7*$day));