Lets say we want to start /home/heather/screenshot.sh every day at 23:58, with the parameter 4.

• Create a service file /etc/systemd/system/HeatherScreenshot.service:

  [Unit]
  Description=HeatherScreenshot
  
  [Service]
  User=heather
  Type=simple
  ExecStart=/home/heather/screenshot.sh 4

• Create a timer file /etc/systemd/system/HeatherScreenshot.timer:

  [Unit]
  Description=Runs HeatherScreenshot
  
  [Timer]
  # DayOfWeek Year-Month-Day Hour:Minute:Second
  OnCalendar=23:58:00
  Unit=HeatherScreenshot.service
  
  [Install]
  WantedBy=multi-user.target

• Test run the static service:

  systemctl start HeatherScreenshot.service

• Start and enable the timer:

  systemctl daemon-reload # if you modified the files
  systemctl start HeatherScreenshot.timer
  systemctl enable HeatherScreenshot.timer

• Show timer status:

  systemctl list-timers --all
  NEXT                         LEFT          LAST                         PASSED       UNIT                         ACTIVATES
  Fri 2016-12-30 22:47:15 CET  4h 35min left Thu 2016-12-29 22:47:15 CET  19h ago      systemd-tmpfiles-clean.timer systemd-tmpfiles-clean.service
  Sat 2016-12-31 00:00:00 CET  5h 47min left Fri 2016-12-30 00:00:05 CET  18h ago      logrotate.timer              logrotate.service
  Sat 2016-12-31 00:00:00 CET  5h 47min left Fri 2016-12-30 00:00:05 CET  18h ago      man-db.timer                 man-db.service
  Sat 2016-12-31 00:00:00 CET  5h 47min left Fri 2016-12-30 00:00:05 CET  18h ago      shadow.timer                 shadow.service
  Sat 2016-12-31 00:59:55 CET  6h left       Fri 2016-12-30 12:27:32 CET  5h 44min ago HeatherScreenshot.timer      HeatherScreenshot.service
  
  5 timers listed.

• To disable a timer, first stop and than disable it:

  systemctl stop HeatherScreenshot.timer
  systemctl disable HeatherScreenshot.timer