I'd probably do
list($year, $month, $day, $hour) = explode(':', date('Y:n:d:h', time()+172800));
$twodays = mktime($hour, 0, 0, $month, $day, $year);
//or if you want it to round to the nearest hour
list($year, $month, $day, $hour, $minute) = explode(':', date('Y:n:d:h:i', time()+172800));
$hour += $minute > 30 ? 1 : 0;
$twodays = mktime($hour, 0, 0, $month, $day, $year);
HTH