crontab → systemd
Paste a cron line and its command; get a ready-to-use .timer + .service pair with a correct OnCalendar= — including the trap that trips most converters: cron's day-of-month/day-of-week OR becomes two OnCalendar= lines, because systemd has no OR within one expression.
Meaning: At 09:00, Monday through Friday
backup.timer
[Unit]
Description=Timer for backup
[Timer]
OnCalendar=Mon..Fri *-*-* 09:00:00
Persistent=true
[Install]
WantedBy=timers.target
backup.service
[Unit]
Description=Job for backup
[Service]
Type=oneshot
ExecStart=/usr/bin/backup.sh
Install & verify
# Save as /etc/systemd/system/backup.timer and /etc/systemd/system/backup.service
sudo systemctl daemon-reload
sudo systemctl enable --now backup.timer
systemctl list-timers backup.timer # verify