diff --git a/.gitignore b/.gitignore
index d69a4c0..8b7abdc 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,5 @@
 deno.d.ts
 .vscode
 .local
+.output
+web/*.js
diff --git a/deps.testing.ts b/deps.testing.ts
new file mode 100644
index 0000000..878ad92
--- /dev/null
+++ b/deps.testing.ts
@@ -0,0 +1,6 @@
+export {
+  describe,
+  expect,
+  it,
+  mockfn,
+} from "https://js.thunderk.net/testing@1.0.0/mod.ts";
diff --git a/src/run.test.ts b/src/run.test.ts
index e69de29..8b11f66 100644
--- a/src/run.test.ts
+++ b/src/run.test.ts
@@ -0,0 +1,29 @@
+import { Sys } from "../deps.ts";
+import { describe, expect, it, mockfn } from "../deps.testing.ts";
+import { Runner } from "./run.ts";
+
+describe(Runner, () => {
+  it("works with a file timestamp", async () => {
+    async function check(stat: any, expected: boolean) {
+      const sys = {
+        stat: mockfn(() => stat),
+        writeTextFile: mockfn(),
+      };
+      const runner = new Runner(sys as typeof Sys);
+      const path = "/tmp/thunderk-run-run-test.stamp";
+
+      const result = await runner.checkStamp("run-test");
+      expect(result).toBe(expected);
+      expect(sys.stat).toHaveBeenLastCalledWith(path);
+      if (expected) {
+        expect(sys.writeTextFile).not.toHaveBeenCalled();
+      } else {
+        expect(sys.writeTextFile).toHaveBeenLastCalledWith(path, "");
+      }
+    }
+
+    await check({ isFile: false }, false);
+    await check({ isFile: true, mtime: new Date(2000, 1, 1) }, false);
+    await check({ isFile: true, mtime: new Date() }, true);
+  });
+});