diff --git a/src/rules_clojure/gen_build.clj b/src/rules_clojure/gen_build.clj
index 3ff8a46..da163ff 100644
--- a/src/rules_clojure/gen_build.clj
+++ b/src/rules_clojure/gen_build.clj
@@ -660,60 +660,71 @@
   "given a source directory, write a BUILD.bazel for all .clj files in the directory. non-recursive"
   [{:keys [deps-edn-dir] :as args} dir]
   (assert (map? (:src-ns->label args)))
-  (let [paths (->> (fs/ls dir)
+  (let [entries (fs/ls dir)
+        paths (->> entries
                    (filter (fn [p]
                              (or (clj*-path? p)
                                  (js-path? p))))
                    (sort-by str))
-        subdirs (->> dir
-                     fs/ls
+        build-file (-> dir (fs/->path "BUILD.bazel") fs/path->file)
+        build-mtime (when (.exists build-file) (.lastModified build-file))
+        subdirs (->> entries
                      (filter (fn [^Path p]
                                (-> p .toFile fs/directory?)))
                      (sort-by str))
-        clj-subdirs (->>
-                     subdirs
-                     (filter (fn [p]
-                               (some clj*-path? (seq (fs/ls-r p))))))
-        [has-binary? rules] (reduce (fn [[hb? acc] rule]
-                                      [(or hb? (= :clojure_binary (:type rule)))
-                                       (conj acc (emit-rule rule))])
-                                    [false []]
-                                    (->> paths
-                                         (group-by fs/basename)
-                                         (mapcat (fn [[_base paths]]
-                                                   (ns-rules args paths)))))
-
-        content (str "#autogenerated, do not edit\n"
-                     (emit-bazel (list 'package (kwargs {:default_visibility ["//visibility:public"]})))
-                     "\n"
-                     (emit-bazel (apply list 'load "@rules_clojure//:rules.bzl"
-                                        (cond-> ["clojure_library" "clojure_test"]
-                                          has-binary? (conj "clojure_binary"))))
-                     "\n"
-                     "\n"
-                     (str/join "\n\n" rules)
-                     "\n"
-                     "\n"
-                     (when (or (seq paths) (seq clj-subdirs))
-                       (str
-                        (emit-bazel (list 'clojure_library (kwargs {:name "__clj_lib"
-                                                                    :deps (vec
-                                                                           (concat
-                                                                            (dedupe (map (fn [p] (str ":" (fs/basename p))) paths))
-                                                                            (map (fn [p]
-                                                                                   (str "//" (fs/path-relative-to deps-edn-dir p) ":__clj_lib")) clj-subdirs)))})))
-                        "\n"
-                        "\n"
-                        (emit-bazel (list 'filegroup (kwargs (merge
-                                                              {:name "__clj_files"
-                                                               :srcs (mapv fs/filename paths)
-                                                               :data (mapv (fn [p]
-                                                                             (str "//" (fs/path-relative-to deps-edn-dir p) ":__clj_files")) clj-subdirs)})))))))]
-    (let [build-file (-> dir (fs/->path "BUILD.bazel") fs/path->file)
-          changed? (not= content (when (.exists build-file) (slurp build-file :encoding "UTF-8")))]
-      (when changed?
-        (spit build-file content :encoding "UTF-8"))
-      {:files (count paths) :wrote (if changed? 1 0)})))
+        skip? (and build-mtime
+                   (every? (fn [^Path p]
+                             (<= (.lastModified (.toFile p)) ^long build-mtime))
+                           paths)
+                   (every? (fn [^Path p]
+                             (let [child-build (-> p (fs/->path "BUILD.bazel") fs/path->file)]
+                               (or (not (.exists child-build))
+                                   (<= (.lastModified child-build) ^long build-mtime))))
+                           subdirs))]
+    (if skip?
+      {:files (count paths) :wrote 0}
+      (let [clj-subdirs (->>
+                         subdirs
+                         (filter (fn [p]
+                                   (some clj*-path? (seq (fs/ls-r p))))))
+            [has-binary? rules] (reduce (fn [[hb? acc] rule]
+                                          [(or hb? (= :clojure_binary (:type rule)))
+                                           (conj acc (emit-rule rule))])
+                                        [false []]
+                                        (->> paths
+                                             (group-by fs/basename)
+                                             (mapcat (fn [[_base paths]]
+                                                       (ns-rules args paths)))))
+            content (str "#autogenerated, do not edit\n"
+                         (emit-bazel (list 'package (kwargs {:default_visibility ["//visibility:public"]})))
+                         "\n"
+                         (emit-bazel (apply list 'load "@rules_clojure//:rules.bzl"
+                                            (cond-> ["clojure_library" "clojure_test"]
+                                              has-binary? (conj "clojure_binary"))))
+                         "\n"
+                         "\n"
+                         (str/join "\n\n" rules)
+                         "\n"
+                         "\n"
+                         (when (or (seq paths) (seq clj-subdirs))
+                           (str
+                            (emit-bazel (list 'clojure_library (kwargs {:name "__clj_lib"
+                                                                        :deps (vec
+                                                                               (concat
+                                                                                (dedupe (map (fn [p] (str ":" (fs/basename p))) paths))
+                                                                                (map (fn [p]
+                                                                                       (str "//" (fs/path-relative-to deps-edn-dir p) ":__clj_lib")) clj-subdirs)))})))
+                            "\n"
+                            "\n"
+                            (emit-bazel (list 'filegroup (kwargs (merge
+                                                                  {:name "__clj_files"
+                                                                   :srcs (mapv fs/filename paths)
+                                                                   :data (mapv (fn [p]
+                                                                                 (str "//" (fs/path-relative-to deps-edn-dir p) ":__clj_files")) clj-subdirs)})))))))
+            changed? (not= content (when build-mtime (slurp build-file :encoding "UTF-8")))]
+        (when changed?
+          (spit build-file content :encoding "UTF-8"))
+        {:files (count paths) :wrote (if changed? 1 0)}))))
 
 (s/fdef gen-source-paths- :args (s/cat :a (s/keys :req-un [::deps-edn-dir ::src-ns->label ::dep-ns->label ::jar->lib ::deps-repo-tag ::deps-bazel]) :paths (s/coll-of fs/path?)))
 (defn gen-source-paths-