nix.gc: let systemd use any time config

modules/services/nix-gc.nix

@@ -4,6 +4,8 @@ with lib;
 
 let
   cfg = config.nix.gc;
+  darwinIntervals =
+    [ "hourly" "daily" "weekly" "monthly" "semiannually" "annually" ];
 
   mkCalendarInterval = frequency:
     let
@@ -66,21 +68,16 @@ in {
       };
 
       frequency = mkOption {
-        type = types.enum [
-          "hourly"
-          "daily"
-          "weekly"
-          "monthly"
-          "semiannually"
-          "annually"
-        ];
+        type = types.str;
         default = "weekly";
-        example = "monthly";
+        example = "03:15";
         description = ''
-          The frequency at which to run the garbage collector.
+          When to run the Nix garbage collector.
 
-          These enums are based on special expressions from the
-          {manpage}`systemd.time(7)`
+          On Linux this is a string as defined by {manpage}`systemd.time(7)`.
+
+          On Darwin it must be one of: ${toString darwinIntervals}, which are
+          implemented as defined in the manual page above.
         '';
       };
 
@@ -117,6 +114,13 @@ in {
     })
 
     (mkIf pkgs.stdenv.isDarwin {
+      assertions = [{
+        assertion = elem cfg.frequency darwinIntervals;
+        message = "On Darwin nix.gc.frequency must be one of: ${
+            toString darwinIntervals
+          }.";
+      }];
+
       launchd.agents.nix-gc = {
         enable = true;
         config = {