systemd Alarm Clock on NixOS

Include the following snippet in your configuration.nix:

systemd.user.services = {
  dailyalarm = {
    description = "Daily Alarm";
    path = [ pkgs.mpv pkgs.bash ]; #The service runs in it's own isolated environment, 
                                   #so we need to explicitly provide it with whatever 
                                   #the script will need, including a shell with which
                                   #to run the script
    wantedBy = [ "multi-user.target" ]; #This ensures that the service is enabled
                                        #at the end of boot up
    serviceConfig = { Type = "oneshot"; ExecStart = /home/tim/alarm.sh; }; 
        #oneshot, roughly, means that systemd will do it's best to run the service, 
        #but if the time is missed, say because the computer was shutdown, it 
        #won't try at next boot until the time rolls around again.
    startAt = "*-*-* 6:30"; #Alarm runs every day at 6:30AM
    };
  };

Then put whatever you’d like your alarm to do in the referenced shell script. Here’s mine:

#! /usr/bin/env bash

mpv ~/summer.mp3