strtotime

echo date("Y/m/d h:i:s");
echo "<br>";
echo date("Y/m/d h:i:s", strtotime(" - 2 sec"));

echo "<br>";
echo "<br>";
$date = "2018/05/20 12:30:00";
echo date("Y/m/d h:i:s", strtotime("$date -1 day"));
echo "<br>";
echo date("Y/m/d h:i:s", strtotime("$date -10 sec"));