For accurate time, use an NTP server to set system time. Synchronized time facilitates auditing and consistency between expiry dates used in expiration of certificates and security protocols.
From the GUI go to System > Settings > System Time and select Synchronize with NTP Server. By default, this causes FortiOS to synchronize with Fortinet's FortiGuard secure NTP server.
From the CLI you can use one or more different NTP servers:
config system ntp
set type custom
set ntpsync enable
set server <ntp-server-ip>
set server <other-ntp-server-ip>