i have tested with systemd timers, that I know little and I wanted to try, so they may not be complete…
content of: /etc/systemd/system/pbs-client.service
[Unit]
Description=Exec pbs-client on mystore
Wants=pbs-client.timer
[Service]
Environment="PBS_PASSWORD=mypass"
#Environment= for PBS_REPOSITORY
Type=oneshot
ExecStart=/usr/local/sbin/proxmox-backup-client backup root.pxar:/ --repository MYPBS:mystore
[Install]
WantedBy=multi-user.target
and /etc/systemd/system/pbs-client.timer
[Unit]
Description=Run pbs-client daily
[Timer]
OnCalendar=daily
Persistent=true
[Install]
WantedBy=timers.target