add time.timeZone option

default.nix

@@ -16,6 +16,7 @@ let
         ./modules/system/defaults/trackpad.nix
         ./modules/system/etc.nix
         ./modules/system/launchd.nix
+        ./modules/time
         ./modules/nix
         ./modules/nix/nix-darwin.nix
         ./modules/nix/nixpkgs.nix

modules/system/activation-scripts.nix

@@ -61,6 +61,7 @@ in
 
       ${cfg.activationScripts.etc.text}
       ${cfg.activationScripts.launchd.text}
+      ${cfg.activationScripts.time.text}
 
       exit $_status
     '';

modules/time/default.nix

@@ -0,0 +1,46 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  cfg = config.time;
+
+  timeZone = optionalString (cfg.timeZone != null) ''
+    if [ -z $(systemsetup -listtimezones | grep "^ ${cfg.timeZone}$") ]; then
+      echo "${cfg.timeZone} is not a valid timezone. The command 'listtimezones' will show a list of valid time zones." >&2
+      false
+    fi
+    systemsetup -settimezone "${cfg.timeZone}" > /dev/null
+  '';
+
+in
+
+{
+  options = {
+
+    time.timeZone = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      example = "America/New_York";
+      description = ''
+        The time zone used when displaying times and dates. See <link
+        xlink:href="https://en.wikipedia.org/wiki/List_of_tz_database_time_zones"/>
+        or run <command>sudo systemsetup -listtimezones</command>
+        for a comprehensive list of possible values for this setting.
+      '';
+    };
+
+  };
+
+  config = {
+
+    system.activationScripts.time.text = ''
+      # Set defaults
+      echo "configuring time..." >&2
+
+      ${timeZone}
+    '';
+
+  };
+}