system/modules/home-manager/programs/calendar.nix

{ config, lib, ... }:

let
  cfg = config.local.programs.calendar;

  mkCalOpts = name: {
    enable = lib.mkEnableOption name;

    # Custom package doesn't work yet.
    # See: https://github.com/nix-community/home-manager/pull/5428
    # package = lib.mkPackageOption pkgs name { };
  };
in
{
  options.local.programs.calendar = {
    khal = mkCalOpts "khal";
  };

  config = {
    accounts.calendar.basePath = "${config.xdg.dataHome}/calendars";

    programs.khal = lib.mkIf cfg.khal.enable {
      enable = true;
      locale = rec {
        timeformat = "%H:%M";
        dateformat = "%Y-%m-%d";
        longdateformat = dateformat;
        datetimeformat = "${dateformat} ${timeformat}";
        longdatetimeformat = datetimeformat;
      };
      # See: https://khal.readthedocs.io/en/latest/configure.html
      settings = {};
    };
  };
}