Comment by throwaway8941

In case anyone else needs this:

  # systemctl edit units-currency-update.service
  
  [Unit]
  Description=Update units(1) currency rates
  
  [Service]
  Type=oneshot
  Nice=19
  ExecStart=/usr/bin/units_cur
  
  # systemctl edit units-currency-update.timer
  
  [Unit]
  Description=Update units(1) currency rates
  
  [Timer]
  OnCalendar=daily
  AccuracySec=3h
  Persistent=true
  
  [Install]
  WantedBy=timers.target
  
  # systemctl daemon-reload
  # systemctl enable units-currency-update.timer