diff --git a/_config.ts b/_config.ts
index b71da1d5f..61292e569 100644
--- a/_config.ts
+++ b/_config.ts
@@ -554,6 +554,42 @@ site.scopedUpdates(
   (path) => path.startsWith("/api/deno/"),
 );
 
+// During dev, auto-update last_modified frontmatter when a page is edited
+site.addEventListener("beforeUpdate", (event) => {
+  const today = new Date().toISOString().slice(0, 10); // YYYY-MM-DD
+
+  for (const file of event.files ?? []) {
+    if (!/\.(md|mdx)$/.test(file)) continue;
+
+    const path = `./${file}`;
+    let content: string;
+    try {
+      content = Deno.readTextFileSync(path);
+    } catch {
+      continue;
+    }
+
+    if (!content.startsWith("---")) continue;
+
+    const endOfFrontmatter = content.indexOf("\n---", 3);
+    if (endOfFrontmatter === -1) continue;
+
+    const frontmatter = content.slice(0, endOfFrontmatter);
+    const rest = content.slice(endOfFrontmatter);
+
+    const lastModifiedMatch = frontmatter.match(/^last_modified:\s*.+$/m);
+    if (lastModifiedMatch) {
+      const updated = frontmatter.replace(
+        /^last_modified:\s*.+$/m,
+        `last_modified: ${today}`,
+      );
+      if (updated !== frontmatter) {
+        Deno.writeTextFileSync(path, updated + rest);
+      }
+    }
+  }
+});
+
 site.addEventListener("afterStartServer", () => {
   log.warn(
     `${cliNow()} Server available at <green>http://localhost:${site.server.options.port}</green>`,
diff --git a/frontmatter_test.ts b/frontmatter_test.ts
index e8c511981..3be433c6f 100644
--- a/frontmatter_test.ts
+++ b/frontmatter_test.ts
@@ -23,6 +23,85 @@ Deno.test("Frontmatter titles must not contain backticks", async (t) => {
   }
 });
 
+Deno.test("last_modified dates must be valid and up to date", async (t) => {
+  // Build a map of file -> last content-change date using git log.
+  // Uses -G to match only commits that changed non-frontmatter lines,
+  // skipping commits that only added/updated the last_modified field itself.
+  const result = new Deno.Command("git", {
+    args: [
+      "log",
+      "-G",
+      "^(?!last_modified:)",
+      "--pretty=format:%aI",
+      "--name-only",
+      "--diff-filter=ACMR",
+      "HEAD",
+    ],
+    stdout: "piped",
+  }).outputSync();
+
+  const output = new TextDecoder().decode(result.stdout);
+  const gitDates = new Map<string, string>();
+  let currentDate = "";
+
+  for (const line of output.split("\n")) {
+    if (!line) continue;
+    if (/^\d{4}-/.test(line)) {
+      currentDate = line.slice(0, 10); // YYYY-MM-DD
+    } else if (!gitDates.has(line)) {
+      gitDates.set(line, currentDate);
+    }
+  }
+
+  for (const dir of DIRS_TO_CHECK) {
+    for await (const entry of walk(dir, { exts: [".md", ".mdx"] })) {
+      const content = await Deno.readTextFile(entry.path);
+      if (!content.startsWith("---")) continue;
+
+      // Extract the raw last_modified value via regex because YAML parse
+      // auto-converts YYYY-MM-DD strings to Date objects
+      const rawMatch = content.match(/^last_modified:\s*(.+)$/m);
+      if (!rawMatch) continue;
+
+      const dateStr = rawMatch[1].trim();
+
+      await t.step(`${entry.path} has valid last_modified`, () => {
+        // Must be YYYY-MM-DD format
+        assert(
+          /^\d{4}-\d{2}-\d{2}$/.test(dateStr),
+          `Invalid date format "${dateStr}" in ${entry.path}. Expected YYYY-MM-DD.`,
+        );
+
+        // Must parse to a real date
+        const parsed = new Date(dateStr + "T00:00:00Z");
+        assert(
+          !isNaN(parsed.getTime()),
+          `"${dateStr}" is not a valid date in ${entry.path}.`,
+        );
+
+        // Must not be in the future
+        const today = new Date().toISOString().slice(0, 10);
+        assert(
+          dateStr <= today,
+          `last_modified "${dateStr}" is in the future in ${entry.path}.`,
+        );
+
+        // Must match the date of the last content change in git history
+        const relativePath = entry.path.replace(/^\.\//, "");
+        const gitDate = gitDates.get(relativePath);
+        if (gitDate) {
+          assertEquals(
+            dateStr,
+            gitDate,
+            `last_modified "${dateStr}" does not match last content change "${gitDate}" for ${entry.path}. ` +
+              `Run the dev server to auto-update, or manually set last_modified: ${gitDate}.`,
+          );
+        }
+      });
+    }
+  }
+});
+
 Deno.test("CLI command page titles must be just the command name", async (t) => {
   for await (
     const entry of walk("./runtime/reference/cli", { exts: [".md"] })