There are two issues:

1) in the first command, clockdiff reports a host is down when it isn't (ie
it's pingable) ... should the text say the host is down or just that clock
diff cannot get the time from it... ?

2) the man page refers to the -o2 option, but the executable doesn't have
that option ...

CLOCKDIFF(8)             Iputils-ss990107            CLOCKDIFF(8)

NAME
       clockdiff - Measures clock difference between us and <destination>
with 1msec resolution. Without -o option it uses icmp timestamps, with -o
it uses icmp echo with timestamp IP option.

SYNOPSIS
       clockdiff [-o|-o2]
<destination>

[root@test95 /root]# clockdiff prep.ai.mit.edu
gnudist.gnu.org is down
[root@test95 /root]# clockdiff -o2 prep.ai.mit.edu
Usage: clockdiff [-o] <host>
[root@test95 /root]# ping prep.ai.mit.edu
PING prep.ai.mit.edu (198.186.203.18) from 207.175.44.95 : 56(84) bytes of
data.
64 bytes from 198.186.203.18: icmp_seq=0 ttl=51 time=117.1 ms
64 bytes from 198.186.203.18: icmp_seq=1 ttl=51 time=190.1 ms

--- prep.ai.mit.edu ping statistics ---
2 packets transmitted, 2 packets received, 0% packet loss
round-trip min/avg/max = 117.1/153.6/190.1 ms
[root@test95
/root]#