Filter ICMP Timestamp Requests using IPtables

The following commands filter ICMP requests
/sbin/iptables -I INPUT 1 -p ICMP --icmp-type timestamp-request -j DROP

The following commands filter ICMP Reply requests
/sbin/iptables -I INPUT 1 -p ICMP --icmp-type timestamp-reply -j DROP

For autorun place the commands in /etc/rc.conf file or rc.local depending on your operating system.