nixpkgs/nixos/modules/system/boot/shutdown.nix

{
  config,
  lib,
  pkgs,
  ...
}:

with lib;

{

  # This unit saves the value of the system clock to the hardware
  # clock on shutdown.
  systemd.services.save-hwclock = {
    description = "Save Hardware Clock";

    wantedBy = [ "shutdown.target" ];

    unitConfig = {
      DefaultDependencies = false;
      ConditionPathExists = "/dev/rtc";
      ConditionPathIsReadWrite = "/etc/";
    };

    serviceConfig = {
      Type = "oneshot";
      ExecStart = "${pkgs.util-linux}/sbin/hwclock --systohc ${
        if config.time.hardwareClockInLocalTime then "--localtime" else "--utc"
      }";
    };
  };

  boot.kernel.sysctl."kernel.poweroff_cmd" = "${config.systemd.package}/sbin/poweroff";

}