Kerberos does not assume clock synchronization.
Kerberos requires reasonable clock synchronization.
And, as near as I can tell, clock synchronization is not part of the
Kerberos protocol.
"Kerberos requires the clocks of the involved hosts to be synchronized.
The tickets have time availability period and, if the host clock is not
synchronized with the clock of Kerberos server, the authentication will
fail. The default configuration requires that clock times are no more
than 10 minutes apart. In practice, NTP daemons are usually employed to keep the host clocks synchronized."