your getting the wrong date in the second line because the Month needs to come before the day.
$today = date("Y.m.D H:i:s");
$today2 = date("Y.m.D H:i:s", mktime(date("H"),date("i"),date("s"),date("m"),date("d"),date("Y")));
// 3 Months from now
$newdate = date("Y.m.D H:i:s", mktime(date("H"),date("i"),date("s"),date("m") + 3,date("d"),date("Y")));
// 30 minutes from now
$newdate2 = date("Y.m.D H:i:s", mktime(date("H"),date("i")+ 30,date("s"),date("m"),date("d"),date("Y")));