It would seem like ICMP timestamping is quite a useful feature for troubleshooting networks with asymmetric paths.
Replies to icmp(4) timestamp requests are generated by the kernel:
FreeBSD even has no option to disable only the timestamp feature of ICMP (see
case ICMP_TSTAMPin icmp_input())
On OS X, the respective setting is net.inet.icmp.timestamp, which is specifically disabled by default. Why?