diff --git a/static_src/elements/chops/chops-timestamp/chops-timestamp-helpers.test.js b/static_src/elements/chops/chops-timestamp/chops-timestamp-helpers.test.js
new file mode 100644
index 0000000..5fe344b
--- /dev/null
+++ b/static_src/elements/chops/chops-timestamp/chops-timestamp-helpers.test.js
@@ -0,0 +1,112 @@
+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import {assert} from 'chai';
+import {FORMATTER, MS_PER_MONTH, standardTime,
+  relativeTime} from './chops-timestamp-helpers.js';
+import sinon from 'sinon';
+
+// The formatted date strings differ based on time zone and browser, so we can't
+// use static strings for testing. We can't stub out the format method because
+// it's native code and can't be modified. So just use the FORMATTER object.
+
+let clock;
+
+describe('chops-timestamp-helpers', () => {
+  beforeEach(() => {
+    // Set clock to the Epoch.
+    clock = sinon.useFakeTimers({
+      now: new Date(0),
+      shouldAdvanceTime: false,
+    });
+  });
+
+  afterEach(() => {
+    clock.restore();
+  });
+
+  describe('standardTime', () => {
+    it('shows relative timestamp when less than a month ago', () => {
+      const date = new Date();
+      assert.equal(standardTime(date), `${FORMATTER.format(date)} (just now)`);
+    });
+
+    it('no relative time when more than a month in the future', () => {
+      const date = new Date(1548808276 * 1000);
+      assert.equal(standardTime(date), 'Tue, Jan 29, 2019, 4:31 PM PST');
+    });
+
+    it('no relative time when more than a month in the past', () => {
+      // Jan 29, 2019, 4:31 PM PST
+      const now = 1548808276 * 1000;
+      clock.tick(now);
+
+      const date = new Date(now - MS_PER_MONTH);
+      assert.equal(standardTime(date), 'Sun, Dec 30, 2018, 4:31 PM PST');
+    });
+  });
+
+  it('relativeTime future', () => {
+    assert.equal(relativeTime(new Date()), `just now`);
+
+    assert.equal(relativeTime(new Date(59 * 1000)), `just now`);
+
+    assert.equal(relativeTime(new Date(60 * 1000)), `a minute from now`);
+    assert.equal(relativeTime(new Date(2 * 60 * 1000)),
+        `2 minutes from now`);
+    assert.equal(relativeTime(new Date(59 * 60 * 1000)),
+        `59 minutes from now`);
+
+    assert.equal(relativeTime(new Date(60 * 60 * 1000)), `an hour from now`);
+    assert.equal(relativeTime(new Date(2 * 60 * 60 * 1000)),
+        `2 hours from now`);
+    assert.equal(relativeTime(new Date(23 * 60 * 60 * 1000)),
+        `23 hours from now`);
+
+    assert.equal(relativeTime(new Date(24 * 60 * 60 * 1000)),
+        `a day from now`);
+    assert.equal(relativeTime(new Date(2 * 24 * 60 * 60 * 1000)),
+        `2 days from now`);
+    assert.equal(relativeTime(new Date(29 * 24 * 60 * 60 * 1000)),
+        `29 days from now`);
+
+    assert.equal(relativeTime(new Date(30 * 24 * 60 * 60 * 1000)),
+        'Jan 30, 1970');
+  });
+
+  it('relativeTime past', () => {
+    const baseTime = 234234 * 1000;
+
+    clock.tick(baseTime);
+
+    assert.equal(relativeTime(new Date()), `just now`);
+
+    assert.equal(relativeTime(new Date(baseTime - 59 * 1000)),
+        `just now`);
+
+    assert.equal(relativeTime(new Date(baseTime - 60 * 1000)),
+        `a minute ago`);
+    assert.equal(relativeTime(new Date(baseTime - 2 * 60 * 1000)),
+        `2 minutes ago`);
+    assert.equal(relativeTime(new Date(baseTime - 59 * 60 * 1000)),
+        `59 minutes ago`);
+
+    assert.equal(relativeTime(new Date(baseTime - 60 * 60 * 1000)),
+        `an hour ago`);
+    assert.equal(relativeTime(new Date(baseTime - 2 * 60 * 60 * 1000)),
+        `2 hours ago`);
+    assert.equal(relativeTime(new Date(baseTime - 23 * 60 * 60 * 1000)),
+        `23 hours ago`);
+
+    assert.equal(relativeTime(new Date(
+        baseTime - 24 * 60 * 60 * 1000)), `a day ago`);
+    assert.equal(relativeTime(new Date(
+        baseTime - 2 * 24 * 60 * 60 * 1000)), `2 days ago`);
+    assert.equal(relativeTime(new Date(
+        baseTime - 29 * 24 * 60 * 60 * 1000)), `29 days ago`);
+
+    assert.equal(relativeTime(new Date(
+        baseTime - 30 * 24 * 60 * 60 * 1000)), 'Dec 4, 1969');
+  });
+});