ddate: 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/bin/ddate - from package: util-linux-ng

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

On Debian possible default full paths are:

/usr/bin/ddate - from package: ddate

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

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