You can check Quantum, you can config it like cron:

config :quantum, :your_app,
  cron: [
    # Every minute
    "* * * * *":      {"Heartbeat", :send},
    # Every 15 minutes
    "*/15 * * * *":   fn -> System.cmd("rm", ["/tmp/tmp_"]) end,
    # Runs on 18, 20, 22, 0, 2, 4, 6:
    "0 18-6/2 * * *": fn -> :mnesia.backup('/var/backup/mnesia') end,
    # Runs every midnight:
    "@daily":         &Backup.backup/0
  ]

I would calculate the initial_interval to 8AM on init, and then send_after 24 hours every time after that.

One thing to keep in mind when using Process.send_after for something like this is that your app might get restarted in between and that message won’t get send.
You could work around that by scheduling a message again on application startup, though.