diff --git a/scripts/data/blog.mjs b/scripts/data/blog.mjs
index 9c90fd4a..7370f66f 100644
--- a/scripts/data/blog.mjs
+++ b/scripts/data/blog.mjs
@@ -11,6 +11,74 @@ const OUTPUT = join(ROOT, 'generated', 'blog.json');
 
 const titleFromBody = body => body.match(/^#\s+(.+)$/m)?.[1].trim() ?? null;
 
+class InvalidBlogDateError extends Error {
+  constructor(file, display) {
+    super(
+      `Invalid blog date in ${join('pages', 'blog', 'posts', file)}: ${display} ` +
+        '(expected an ISO 8601 date or timestamp)'
+    );
+    this.name = 'InvalidBlogDateError';
+  }
+}
+
+const daysInMonth = (year, month) => {
+  if (month === 2) {
+    return year % 4 === 0 && (year % 100 !== 0 || year % 400 === 0) ? 29 : 28;
+  }
+
+  return [31, 0, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month - 1] ?? 0;
+};
+
+const normalizeDate = (value, file, source) => {
+  const rawDate = source
+    .match(/^---\r?\n([\s\S]*?)\r?\n---(?:\r?\n|$)/)?.[1]
+    ?.match(/^date:\s*(.*?)\s*$/m)?.[1];
+  const calendar = rawDate?.match(
+    /^['"]?(\d{4})-(\d{1,2})-(\d{1,2})(?:(?:[Tt]|[ \t]+)(\d{1,2}):(\d{2})(?::(\d{2}))?)?/
+  );
+
+  if (
+    calendar &&
+    (Number(calendar[2]) < 1 ||
+      Number(calendar[2]) > 12 ||
+      Number(calendar[3]) < 1 ||
+      Number(calendar[3]) >
+        daysInMonth(Number(calendar[1]), Number(calendar[2])) ||
+      (calendar[4] !== undefined &&
+        (Number(calendar[4]) > 24 ||
+          (Number(calendar[4]) === 24 &&
+            (Number(calendar[5]) !== 0 ||
+              (calendar[6] !== undefined && Number(calendar[6]) !== 0))))) ||
+      (calendar[5] !== undefined && Number(calendar[5]) > 59) ||
+      (calendar[6] !== undefined && Number(calendar[6]) > 59))
+  ) {
+    const display = JSON.stringify(rawDate.trim());
+    throw new InvalidBlogDateError(file, display);
+  }
+
+  const date =
+    value instanceof Date
+      ? value
+      : typeof value === 'string' && value.trim()
+        ? new Date(
+            /^\d{4}-\d{1,2}-\d{1,2}(?:[Tt]|[ \t]+)\d{1,2}:\d{2}(?::\d{2}(?:\.\d+)?)?$/.test(
+              value.trim()
+            )
+              ? `${value.trim().replace(/[ \t]+/, 'T')}Z`
+              : value
+          )
+        : null;
+
+  if (!date || Number.isNaN(date.getTime())) {
+    const displayValue = rawDate?.trim() || value;
+    const display =
+      displayValue === undefined ? '<missing>' : JSON.stringify(displayValue);
+    throw new InvalidBlogDateError(file, display);
+  }
+
+  return date.toISOString();
+};
+
 const readPosts = async () => {
   const entries = await readdir(POSTS_DIR);
   const files = entries.filter(name => name.endsWith('.md'));
@@ -18,15 +86,14 @@ const readPosts = async () => {
   const posts = await Promise.all(
     files.map(async file => {
       const slug = file.replace(/\.md$/, '');
-      const { data, content } = matter(
-        await readFile(join(POSTS_DIR, file), 'utf8')
-      );
+      const source = await readFile(join(POSTS_DIR, file), 'utf8');
+      const { data, content } = matter(source);
 
       return {
         slug,
         title: titleFromBody(content) ?? slug,
         authors: data.authors?.split(',').map(s => s.trim()),
-        date: new Date(data.date).toISOString(),
+        date: normalizeDate(data.date, file, source),
         category: data.category ?? null,
         image: data.image ?? null,
         ...(data.description && { description: data.description }),
diff --git a/tests/data/blog.test.mjs b/tests/data/blog.test.mjs
new file mode 100644
index 00000000..4e8ab856
--- /dev/null
+++ b/tests/data/blog.test.mjs
@@ -0,0 +1,151 @@
+import { strict as assert } from 'node:assert';
+import { execFile } from 'node:child_process';
+import {
+  copyFile,
+  mkdtemp,
+  mkdir,
+  readFile,
+  rm,
+  writeFile,
+} from 'node:fs/promises';
+import { promisify } from 'node:util';
+import { fileURLToPath } from 'node:url';
+import { join } from 'node:path';
+import { test } from 'node:test';
+
+const execFileAsync = promisify(execFile);
+const blogScript = fileURLToPath(
+  new URL('../../scripts/data/blog.mjs', import.meta.url)
+);
+
+const createFixture = async content => {
+  const root = await mkdtemp(join('tests', 'data', '.blog-'));
+  await mkdir(join(root, 'scripts', 'data'), { recursive: true });
+  await mkdir(join(root, 'pages', 'blog', 'posts'), { recursive: true });
+  await mkdir(join(root, 'generated'), { recursive: true });
+  await copyFile(blogScript, join(root, 'scripts', 'data', 'blog.mjs'));
+  await writeFile(join(root, 'pages', 'blog', 'posts', 'example.md'), content);
+  return root;
+};
+
+const runFixture = (root, timeZone = 'UTC') =>
+  execFileAsync(process.execPath, ['scripts/data/blog.mjs'], {
+    cwd: root,
+    env: { ...process.env, TZ: timeZone },
+  });
+
+test('generates blog data for valid dates', async t => {
+  const root = await createFixture("---\ndate: '2026-09-22'\n---\n# Test\n");
+  t.after(() => rm(root, { recursive: true, force: true }));
+
+  await runFixture(root);
+  const posts = JSON.parse(
+    await readFile(join(root, 'generated', 'blog.json'), 'utf8')
+  );
+
+  assert.equal(posts[0].date, '2026-09-22T00:00:00.000Z');
+});
+
+test('normalizes offset-less quoted date-times as UTC', async t => {
+  const root = await createFixture(
+    "---\ndate: '2026-09-22 10:00'\n---\n# Test\n"
+  );
+  t.after(() => rm(root, { recursive: true, force: true }));
+
+  const dates = [];
+  for (const timeZone of ['UTC', 'America/Los_Angeles']) {
+    await runFixture(root, timeZone);
+    const posts = JSON.parse(
+      await readFile(join(root, 'generated', 'blog.json'), 'utf8')
+    );
+    dates.push(posts[0].date);
+  }
+
+  assert.deepEqual(dates, [
+    '2026-09-22T10:00:00.000Z',
+    '2026-09-22T10:00:00.000Z',
+  ]);
+});
+
+test('reports the file for an invalid blog date', async t => {
+  const root = await createFixture('---\ndate: not-a-date\n---\n# Test\n');
+  t.after(() => rm(root, { recursive: true, force: true }));
+
+  await assert.rejects(runFixture(root), error => {
+    assert.match(error.stderr, /InvalidBlogDateError: Invalid blog date/);
+    assert.match(error.stderr, /pages[\\/]blog[\\/]posts[\\/]example\.md/);
+    assert.match(error.stderr, /not-a-date/);
+    return true;
+  });
+});
+
+test('rejects nonexistent calendar dates', async t => {
+  const root = await createFixture('---\ndate: 2026-02-30\n---\n# Test\n');
+  t.after(() => rm(root, { recursive: true, force: true }));
+
+  await assert.rejects(runFixture(root), error => {
+    assert.match(error.stderr, /Invalid blog date/);
+    assert.match(error.stderr, /2026-02-30/);
+    return true;
+  });
+});
+
+test('rejects one-digit dates that roll over', async t => {
+  const root = await createFixture(
+    '---\ndate: 2024-2-30 10:00:00\n---\n# Test\n'
+  );
+  t.after(() => rm(root, { recursive: true, force: true }));
+
+  await assert.rejects(runFixture(root), error => {
+    assert.match(error.stderr, /Invalid blog date/);
+    assert.match(error.stderr, /2024-2-30/);
+    return true;
+  });
+});
+
+test('rejects out-of-range time components', async t => {
+  const root = await createFixture(
+    '---\ndate: 2024-01-01 25:60:60\n---\n# Test\n'
+  );
+  t.after(() => rm(root, { recursive: true, force: true }));
+
+  await assert.rejects(runFixture(root), error => {
+    assert.match(error.stderr, /Invalid blog date/);
+    assert.match(error.stderr, /25:60:60/);
+    return true;
+  });
+});
+
+test('accepts ISO end-of-day timestamps', async t => {
+  for (const value of ['2026-09-22T24:00:00Z', '2026-09-22 24:00']) {
+    const root = await createFixture(`---\ndate: '${value}'\n---\n# Test\n`);
+    t.after(() => rm(root, { recursive: true, force: true }));
+
+    await runFixture(root);
+  }
+});
+
+test('rejects nonzero end-of-day time components', async t => {
+  const root = await createFixture(
+    "---\ndate: '2026-09-22T24:00:01Z'\n---\n# Test\n"
+  );
+  t.after(() => rm(root, { recursive: true, force: true }));
+
+  await assert.rejects(runFixture(root), error => {
+    assert.match(error.stderr, /Invalid blog date/);
+    assert.match(error.stderr, /24:00:01/);
+    return true;
+  });
+});
+
+test('reports the file for a missing blog date', async t => {
+  const root = await createFixture('---\ntitle: Test\n---\n# Test\n');
+  t.after(() => rm(root, { recursive: true, force: true }));
+
+  await assert.rejects(runFixture(root), error => {
+    assert.match(error.stderr, /Invalid blog date/);
+    assert.match(error.stderr, /example\.md/);
+    assert.match(error.stderr, /<missing>/);
+    return true;
+  });
+});