diff --git a/tools/update_copyright.janet b/tools/update_copyright.janet
new file mode 100644
index 00000000..04758fb6
--- /dev/null
+++ b/tools/update_copyright.janet
@@ -0,0 +1,44 @@
+(def usage (string "usage: janet " (first (dyn :args)) " <last-year> <this-year>"))
+
+(def ignores [".git"])
+(def exts [".c" ".h" ".janet"])
+
+(defn arg [i]
+  (defn bail [] (print usage) (quit))
+  (if-not (= 3 (length (dyn :args)))
+    (bail)
+    (if-let [val (get (dyn :args) i)]
+      val
+      (bail))))
+
+(def oy (arg 1))
+(def ny (arg 2))
+(def od (string "Copyright (c) " oy " Calvin Rose"))
+(def nd (string "Copyright (c) " ny " Calvin Rose"))
+
+(defn join [dir name]
+  (os/realpath (string dir "/" name)))
+
+(defn add-children [dir paths]
+  (loop [name :in (os/dir dir)
+              :unless (has-value? ignores name)]
+   (array/push paths (join dir name))))
+
+(defn ends-in? [exts s]
+  (find (fn [ext] (string/has-suffix? ext s)) exts))
+
+(defn update-disclaimer [path]
+  (if-let [_   (ends-in? exts path)
+           oc  (slurp path)
+           pos (string/find od oc)
+           nc  (string (string/slice oc 0 pos) nd (string/slice oc (+ pos (length od))))]
+    (spit path nc)))
+
+(def cwd (os/cwd))
+(def paths (if (string/has-suffix? "janet" cwd)
+             @[cwd]
+             @[(join cwd "..")]))
+(loop [p :in paths]
+  (if (= :directory ((os/stat p) :mode))
+    (add-children p paths)
+    (update-disclaimer p)))