If you already have changed all the necessary configuration files to show the new hostname you can reset the hostname just by executing:
Then when you log out and back in again, the new hostname will be shown at the prompt. If the hostname is to be used by services like Nagios, then restart the service itself. There's no need whatsoever to reboot an entire server to change the hostname. Also, restarting a service or resetting the hostname for as far as I know has nothing to do and should not affect your date and time settings. Why do you refer to date and time? Does your server show the correct time after a reboot?