You set the system time in the Settings screen of the Device | Settings > Time page.
To set the system time
Click away from the calendar to accept the settings.
To use the manually entered list of NTP servers to set the firewall clock rather than the internal list of NTP servers, select Only use custom NTP servers.
Select this option only if you have configured one or more NTP servers. For more information about NTP servers, see Configuring NTP Settings.