now you need to modify the operating system time with PHP. The system is currently centos

my code

<?php
//$cmd = "date -s "20181123 08:00:00"";
//$cmd = "timedatectl set-time 2018-11-23 08:00:00";
$cmd = "reboot";
echo date("Y-m-d H:i:s")."<br/>";
$output = shell_exec($cmd);
shell_exec("hwclock --systohc --localtime");
shell_exec("clock w");
echo "
";
echo $output;
echo "========================"."<br/>";
echo date("Y-m-d H:i:s")."<br/>";
?>
 the situation in the process 
 first I have confirmed that the server can execute the function shell_exec. For example, there is no problem with executing shell_exec ("ls-a"). 
 neither changing the system time nor rebooting the system seems to work. 
 the bosses who want to know can help to see or analyze what the problem is.