diff --git a/blog/tests/jsonLD.test.ts b/blog/tests/jsonLD.test.ts
new file mode 100644
index 000000000..c6e83b1dc
--- /dev/null
+++ b/blog/tests/jsonLD.test.ts
@@ -0,0 +1,78 @@
+import { assertEquals } from "@std/assert";
+import { BlogPost } from "../types.ts";
+import { toISODateTime } from "../utils/date.ts";
+import { toBlogPosting } from "../utils/jsonLD.ts";
+
+const post = (overrides: Partial<BlogPost> = {}): BlogPost => ({
+  title: "How to brew",
+  slug: "how-to-brew",
+  date: "2024-06-01",
+  excerpt: "A guide",
+  ...overrides,
+});
+
+Deno.test("toISODateTime places a bare date at 08:00 UTC", () => {
+  assertEquals(toISODateTime("2025-06-01"), "2025-06-01T08:00:00.000Z");
+});
+
+Deno.test("toISODateTime pins an offset-less date-time to UTC", () => {
+  assertEquals(
+    toISODateTime("2025-06-01T00:00:00"),
+    "2025-06-01T00:00:00.000Z",
+  );
+});
+
+Deno.test("toISODateTime keeps the instant of an offset date-time", () => {
+  assertEquals(
+    toISODateTime("2025-06-01T09:00:00+02:00"),
+    "2025-06-01T07:00:00.000Z",
+  );
+});
+
+Deno.test("toISODateTime keeps the Unix epoch", () => {
+  assertEquals(
+    toISODateTime("1970-01-01T00:00:00Z"),
+    "1970-01-01T00:00:00.000Z",
+  );
+});
+
+Deno.test("toISODateTime returns undefined for an unparseable value", () => {
+  assertEquals(toISODateTime(""), undefined);
+  assertEquals(toISODateTime("not a date"), undefined);
+});
+
+Deno.test("toISODateTime rejects an impossible calendar date", () => {
+  assertEquals(toISODateTime("2024-02-31"), undefined);
+  assertEquals(toISODateTime("2024-02-31T10:00:00Z"), undefined);
+});
+
+Deno.test("toISODateTime rejects non-ISO strings", () => {
+  assertEquals(toISODateTime("June 1, 2025"), undefined);
+});
+
+Deno.test("toBlogPosting normalizes full timestamps and bare dates alike", () => {
+  assertEquals(
+    toBlogPosting(post({ date: "2025-10-31T15:10:01Z" })).datePublished,
+    "2025-10-31T15:10:01.000Z",
+  );
+  assertEquals(
+    toBlogPosting(post({ date: "2026-08-14" })).datePublished,
+    "2026-08-14T08:00:00.000Z",
+  );
+});
+
+Deno.test("toBlogPosting emits dates with a timezone", () => {
+  const node = toBlogPosting(post({ dateModified: "2024-07-01T10:30:00" }));
+
+  assertEquals(node.datePublished, "2024-06-01T08:00:00.000Z");
+  assertEquals(node.dateModified, "2024-07-01T10:30:00.000Z");
+});
+
+Deno.test("toBlogPosting drops dates that can't be parsed", () => {
+  const node = toBlogPosting(
+    post({ date: "not a date", dateModified: "soon" }),
+  );
+
+  assertEquals("datePublished" in node, false);
+  assertEquals("dateModified" in node, false);
+});
diff --git a/blog/utils/date.ts b/blog/utils/date.ts
index 1299df9db..8d1fe55e7 100644
--- a/blog/utils/date.ts
+++ b/blog/utils/date.ts
@@ -94,3 +94,28 @@ export const scheduledTime = (value: string): number | null => {
   // `+99:00` matches the pattern but is not a real instant.
   return Number.isNaN(parsed) ? null : parsed;
 };
+
+const BARE_DATE = /^\d{4}-\d{2}-\d{2}$/;
+
+/**
+ * Normalizes a CMS-authored date into a full ISO 8601 date-time with a
+ * timezone, for JSON-LD's `datePublished`/`dateModified`, or returns
+ * `undefined` if the value isn't a real ISO date.
+ *
+ * `BlogPost.date` is usually a bare `YYYY-MM-DD` (the admin's date picker),
+ * which Google's Rich Results Test rejects as an invalid datetime missing its
+ * timezone. A bare date carries no time of day, so it is placed at 08:00 UTC
+ * rather than midnight: that instant falls on the same calendar day from
+ * UTC-08:00 to UTC+14:00, so search results don't show a post a day early in
+ * the Americas.
+ *
+ * Parsing goes through `scheduledTime` rather than `dateToTime` for the same
+ * reasons it exists: an impossible date such as `2024-02-31` is dropped instead
+ * of rolling over into March, non-ISO strings aren't read in server-local time,
+ * and the Unix epoch isn't mistaken for a failure. Anything rejected is omitted
+ * rather than emitted as invalid structured data.
+ */
+export const toISODateTime = (date: string) => {
+  const time = scheduledTime(BARE_DATE.test(date) ? `${date}T08:00:00` : date);
+  return time === null ? undefined : new Date(time).toISOString();
+};
diff --git a/blog/utils/jsonLD.ts b/blog/utils/jsonLD.ts
index 0c2c8cac8..43a4ede54 100644
--- a/blog/utils/jsonLD.ts
+++ b/blog/utils/jsonLD.ts
@@ -1,4 +1,5 @@
 import { Author, BlogPost, Category, Publisher } from "../types.ts";
+import { toISODateTime } from "./date.ts";
 
 const toAuthor = (author: Author) => {
   const type = author.type ?? "Person";
@@ -76,13 +77,18 @@ export const toBlogPosting = (
       ? post.interactionStatistic
       : undefined;
 
+  const datePublished = post.date ? toISODateTime(post.date) : undefined;
+  const dateModified = post.dateModified
+    ? toISODateTime(post.dateModified)
+    : undefined;
+
   return {
     "@type": "BlogPosting" as const,
     headline: post.title,
     ...(post.excerpt ? { description: post.excerpt } : {}),
     ...(image ? { image: [image] } : {}),
-    ...(post.date ? { datePublished: post.date } : {}),
-    ...(post.dateModified ? { dateModified: post.dateModified } : {}),
+    ...(datePublished ? { datePublished } : {}),
+    ...(dateModified ? { dateModified } : {}),
     ...(post.authors?.length ? { author: post.authors.map(toAuthor) } : {}),
     ...(publisher?.name ? { publisher: toOrganization(publisher) } : {}),
     ...(categories?.length ? { articleSection: categories } : {}),