zfs/etc/systemd/system/zfs-scrub-monthly@.timer.in

[Unit]
Description=Monthly zpool scrub timer for %i
Documentation=man:zpool-scrub(8)

[Timer]
OnCalendar=monthly
Persistent=true
RandomizedDelaySec=1h
Unit=zfs-scrub@%i.service

[Install]
WantedBy=timers.target