clockdiff: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/sbin/clockdiff - from package: iputils

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/clockdiff - from package: iputils-clockdiff

You may install the required package with command apt-get install {package}

Error in other languages:
clockdiff: bevel nie gevind nie
clockdiff: comandă negăsită
clockdiff: comando não encontrado
clockdiff: commande introuvable
clockdiff: command not found
clockdiff: käsku ei ole
clockdiff: không tìm thấy lệnh
clockdiff: komanda nerasta
clockdiff: Kommando nicht gefunden.
clockdiff: kommandot finns inte
clockdiff: komut yok
clockdiff: nie znaleziono polecenia
clockdiff: níor aimsíodh an t-ordú
clockdiff: no se encontró la orden
clockdiff: no s'ha trobat l'ordre
clockdiff: opdracht niet gevonden
clockdiff: parancs nem található
clockdiff: perintah tidak ditemukan
clockdiff: príkaz nenájdený
clockdiff: příkaz nenalezen
clockdiff: команда не найдена
clockdiff: командата не е открита
clockdiff: 命令找不到
clockdiff: コマンドが見つかりません