[Unit]
Description=Run backup.sh at specified interval
RefuseManualStart=no        # Allow manual starts
RefuseManualStop=no         # Allow manual stops

[Timer]
# Execute job if it missed a run due to machine being off
Persistent=true
# Run 15 minutes after boot for the first time
OnBootSec=900
# Run every 24 hours thereafter
OnUnitInactiveSec=86400
# File describing job to execute
Unit=backup.sh.service

[Install]
WantedBy=timers.target