diff --git a/www/build.sh b/www/build.sh
index a80c62b..2f221d5 100755
--- a/www/build.sh
+++ b/www/build.sh
@@ -1,6 +1,7 @@
 #!/bin/bash
 mkdir data
 (cd src; hy build.hy)
+rm -rf site/html
 mkdir -p site/{html,assets,scripts}
 cp -r src/output/* site/html
 echo 'html generated'
diff --git a/www/src/build.hy b/www/src/build.hy
index 359fdbd..10efad7 100644
--- a/www/src/build.hy
+++ b/www/src/build.hy
@@ -1,5 +1,6 @@
 (import glob [glob])
-(import os [mkdir])
+(import subprocess [check-output])
+(import os [mkdir stat])
 (import os.path [isfile :as file? isdir :as dir?])
 (import templates *)
 (import utils *)
@@ -7,6 +8,7 @@
 (import hy.compiler [hy-eval])
 
 (require hyrule.control [defmain])
+(require hyrule.argmove [->])
 
 (setv local-scope (locals))
 (setv global-scope (globals))
@@ -19,11 +21,22 @@
   (lfor name names
     `(setv (get local-scope (str (get '(~name) 0))) ~name)))
 
+(defn last-changed [file]
+  (getattr (stat file) "st_ctime"))
+
+(defn last-changed-prev-compile [file]
+  (getattr (stat (-> file (.replace ".hy" ".html") (.replace "./pages/" "../site/html/"))) "st_ctime"))
+
 (defmain []
+  (setv last-compile-time (last-changed "../site"))
+  (setv data-dir-changed? (> (last-changed "data") last-compile-time))
+  (setv last-commit-newer (> (-> (check-output #[[git log -1 --format="%at"]] :shell True) (.decode) (.strip) (int)) last-compile-time))
+
+  (print f"last compiled: {last-compile-time :.0f}")
+
   (when (not (dir? "output"))
     (mkdir "output"))
 
-
   (for [path (glob "./pages/**/*" :recursive True)]
     (when (in "__pycache__" path)
       (continue))
@@ -34,14 +47,22 @@
           (print f"creating {path}")
           (mkdir (.replace path "pages" "output")))
 
-      (file? path) 
-        (do 
-          (print f"building {path}")
-          (setv page-name (.split (cut path 2 -3) "/"))
-          (enshrine page-name)
-          (with [target (open (+ "./output/" (.join "/" (cut page-name 1 None)) ".html") "w")] 
-            (.write target (form->html
-              (hy-eval (hy.read (with [source (open path "r")]
-                (.read source)))
-                :locals local-scope
-                :globals global-scope))))))))
+      (file? path)
+        ;; only recompile pages when:
+        ;;  - the source page is newer than the output
+        ;;  - the `src/data` directory is newer than the output
+        ;;  - the latest commit is newer than the output
+        (if (or (> (last-changed path) (last-changed-prev-compile path)) data-dir-changed? last-commit-newer)
+          (do 
+            (print f"building {path}")
+            (setv page-name (.split (cut path 2 -3) "/"))
+            (enshrine page-name)
+            (with [target (open (+ "./output/" (.join "/" (cut page-name 1 None)) ".html") "w")] 
+              (.write target (form->html
+                (hy-eval (hy.read (with [source (open path "r")]
+                  (.read source)))
+                  :locals local-scope
+                  :globals global-scope)))))
+          
+          (print f"{(.replace path #[[./pages/]] #[[../site/html/]])} more recent, skipping")))))
+