diff --git a/eleventy.config.js b/eleventy.config.js
index 333536b..44d641b 100644
--- a/eleventy.config.js
+++ b/eleventy.config.js
@@ -6,6 +6,12 @@ import { postComparator, isPostPublic } from "./src/_utils/sort-posts.js";
 import { validateLinkedArticles } from "./src/_utils/linked-articles.js";
 
 export default function (eleventyConfig) {
+  let buildTimestamp = new Date().toISOString();
+  eleventyConfig.on("eleventy.before", () => {
+    buildTimestamp = new Date().toISOString();
+  });
+  eleventyConfig.addGlobalData("buildTimestamp", () => buildTimestamp);
+
   // Build ignored local post fixtures without making them Git-tracked.
   eleventyConfig.setUseGitIgnore(false);
 
diff --git a/src/_includes/layouts/base.njk b/src/_includes/layouts/base.njk
index 65a0a92..5d80f5d 100644
--- a/src/_includes/layouts/base.njk
+++ b/src/_includes/layouts/base.njk
@@ -29,6 +29,7 @@
     {% include "partials/footer.njk" %}
   </div>
 
+  <script type="module" src="/assets/js/build-timer.js"></script>
   {% block scripts %}{% endblock %}
 </body>
 </html>
diff --git a/src/_includes/partials/footer.njk b/src/_includes/partials/footer.njk
index 684c2d5..1f7d1eb 100644
--- a/src/_includes/partials/footer.njk
+++ b/src/_includes/partials/footer.njk
@@ -4,6 +4,8 @@
     <span>RUNTIME: ELEVENTY 3.x STATIC</span>
     <span>&bull;</span>
     <span>THEME: NATIVE TERMINAL DARK</span>
+    <span>&bull;</span>
+    <time id="build-age" data-build-timestamp="{{ buildTimestamp }}" aria-live="polite">REBUILT: INITIALIZING</time>
   </div>
 
   <div class="footer-right">
diff --git a/src/assets/js/build-timer.js b/src/assets/js/build-timer.js
new file mode 100644
index 0000000..814aad3
--- /dev/null
+++ b/src/assets/js/build-timer.js
@@ -0,0 +1,30 @@
+export const formatBuildAge = (buildTimestamp, nowTimestamp = new Date().toISOString()) => {
+  const buildTime = Date.parse(buildTimestamp);
+  const now = Date.parse(nowTimestamp);
+
+  if (!Number.isFinite(buildTime) || !Number.isFinite(now)) return "unknown";
+
+  const elapsedSeconds = Math.max(0, Math.floor((now - buildTime) / 1000));
+  const days = Math.floor(elapsedSeconds / 86400);
+  const hours = Math.floor((elapsedSeconds % 86400) / 3600);
+  const minutes = Math.floor((elapsedSeconds % 3600) / 60);
+  const seconds = elapsedSeconds % 60;
+
+  if (days > 0) return `${days}d ${hours}h ago`;
+  if (hours > 0) return `${hours}h ${minutes}m ago`;
+  if (minutes > 0) return `${minutes}m ${seconds}s ago`;
+  return `${seconds}s ago`;
+};
+
+if (typeof document !== "undefined") {
+  const timer = document.getElementById("build-age");
+
+  if (timer) {
+    const updateBuildAge = () => {
+      timer.textContent = `REBUILT: ${formatBuildAge(timer.dataset.buildTimestamp)}`;
+    };
+
+    updateBuildAge();
+    window.setInterval(updateBuildAge, 1000);
+  }
+}
diff --git a/test/build-age.test.js b/test/build-age.test.js
new file mode 100644
index 0000000..6ad7161
--- /dev/null
+++ b/test/build-age.test.js
@@ -0,0 +1,62 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import path from "node:path";
+import configureEleventy from "../eleventy.config.js";
+import { formatBuildAge } from "../src/assets/js/build-timer.js";
+
+test("Build age: formats elapsed time as a compact live footer value", () => {
+  assert.equal(
+    formatBuildAge("2026-10-20T12:00:00.000Z", "2026-10-20T12:00:00.000Z"),
+    "0s ago"
+  );
+  assert.equal(
+    formatBuildAge("2026-10-20T12:00:00.000Z", "2026-10-20T12:01:01.000Z"),
+    "1m 1s ago"
+  );
+  assert.equal(
+    formatBuildAge("2026-10-20T12:00:00.000Z", "2026-10-21T14:03:04.000Z"),
+    "1d 2h ago"
+  );
+  assert.equal(
+    formatBuildAge("2026-10-20T12:00:00.000Z", "2026-10-20T11:59:59.000Z"),
+    "0s ago"
+  );
+});
+
+test("Build age: Eleventy exposes one ISO timestamp for each rebuild", () => {
+  const globalData = new Map();
+  const events = new Map();
+  configureEleventy({
+    setUseGitIgnore() {},
+    addPassthroughCopy() {},
+    addPlugin() {},
+    setLibrary() {},
+    addFilter() {},
+    addCollection() {},
+    addGlobalData(name, value) {
+      globalData.set(name, value);
+    },
+    on(name, callback) {
+      events.set(name, callback);
+    },
+    setServerOptions() {}
+  });
+
+  events.get("eleventy.before")();
+  assert.match(
+    globalData.get("buildTimestamp")(),
+    /^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}Z$/
+  );
+});
+
+test("Build age: footer provides an accessible live timer backed by the build timestamp", () => {
+  const footer = fs.readFileSync(path.resolve("src/_includes/partials/footer.njk"), "utf-8");
+  const baseLayout = fs.readFileSync(path.resolve("src/_includes/layouts/base.njk"), "utf-8");
+
+  assert.match(
+    footer,
+    /<time id="build-age" data-build-timestamp="{{ buildTimestamp }}" aria-live="polite">REBUILT: INITIALIZING<\/time>/
+  );
+  assert.match(baseLayout, /<script type="module" src="\/assets\/js\/build-timer\.js"><\/script>/);
+});
diff --git a/test/linked-articles.test.js b/test/linked-articles.test.js
index ffa8ef7..0423f64 100644
--- a/test/linked-articles.test.js
+++ b/test/linked-articles.test.js
@@ -95,6 +95,8 @@ test("Linked articles: the Eleventy posts collection rejects invalid frontmatter
     addCollection(name, callback) {
       collections.set(name, callback);
     },
+    addGlobalData() {},
+    on() {},
     setServerOptions() {}
   });