diff --git a/modules/base.nix b/modules/base.nix
index 8f9f6df..61d8f7c 100644
--- a/modules/base.nix
+++ b/modules/base.nix
@@ -83,7 +83,12 @@
   nix.gc = {
     automatic = true;
     dates = "weekly";
-    options = "--delete-older-than 7d";
+    # Options given to nix-collect-garbage.
+    # They are documented here: https://nixos.org/manual/nix/stable/command-ref/nix-collect-garbage
+    # The `--delete-older-than <period` option refers the manual for `nix-env --delete-generations <period>`: https://nixos.org/manual/nix/stable/command-ref/nix-env/delete-generations#generations-time
+    # There, two meanings of <period> are described. The second one is:
+    #   +<number>: Keep the last number generations, along with any newer than current.
+    options = "--delete-older-than +7";
   };
 
   # Delete all files in /tmp during boot.