diff --git a/ops/nixos/lib/common.nix b/ops/nixos/lib/common.nix
index 6418855063..1cf194396f 100644
--- a/ops/nixos/lib/common.nix
+++ b/ops/nixos/lib/common.nix
@@ -4,10 +4,15 @@
 
 { pkgs, config, depot, lib, rebuilder, ... }@args:
 let 
-  inherit (lib) mkDefault;
   inherit (depot.ops) secrets;
 
   switch-prebuilt = import ./switch-prebuilt.nix args;
+
+  # Default default priority is 1000; we use 900 here so we're higher priority,
+  # but lower priority than user-specified things.  This is particularly
+  # important for our timezone setting, which otherwise conflicts with the one
+  # set by the Clickhouse module, which is used by e.g. bvm-logger.
+  mkDefault = lib.mkOverride 900;
 in
 {
   imports = [ ../../../third_party/home-manager/nixos ];
@@ -197,7 +202,7 @@ in
 
     # Clean up daily.
     nix.gc = {
-      automatic = lib.mkDefault true;
+      automatic = mkDefault true;
       dates = "*-*-* 05:00:00";
       options = "--delete-older-than 7d";
     };