You should not allow root SSH access, period. Instead log in as unprivileged user then use Sudo to perform tasks that require root account privileges.
If you want to allow time-based root logins regardless then at least
AND harden your sshd_config
AND implement one method of
http://www.linuxquestions.org/questi...d.php?t=340366 AND restrict access of remote IP's using your firewall and tcp_wrappers.
When you've done that look at PAM_time's /etc/security/time.conf.