diff --git a/web/src/lib/journal.test.ts b/web/src/lib/journal.test.ts
new file mode 100644
index 000000000..5d198a067
--- /dev/null
+++ b/web/src/lib/journal.test.ts
@@ -0,0 +1,44 @@
+// Every journal date is a Seoul time ("2026-05-14T01:30:00+09:00"), and the
+// pages are built wherever the build runs. On 27 October a build in UTC printed
+// 60 of the 520 dates on /journal a day early: headache, stopped,
+// i-hope-it-works-out and negative were written between midnight and 9 a.m. in
+// Seoul, which is still the day before in UTC.
+
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { formatJournalDate, getAllJournalPosts, type JournalLang } from "./journal";
+
+// UTC is a build server, Los Angeles is nine hours further behind it, and
+// Kiritimati (UTC+14) is the one that runs ahead of Seoul.
+const ZONES = ["UTC", "America/Los_Angeles", "Pacific/Kiritimati", "Asia/Seoul"];
+
+afterEach(() => {
+  vi.unstubAllEnvs();
+});
+
+describe("formatJournalDate", () => {
+  it.each(ZONES)("prints the Seoul day when the process runs in %s", (zone) => {
+    vi.stubEnv("TZ", zone);
+
+    expect(formatJournalDate("2026-05-14T01:30:00+09:00", "ko")).toBe("2026년 5월 14일");
+    expect(formatJournalDate("2026-05-14T01:30:00+09:00", "en")).toBe("May 14, 2026");
+    expect(formatJournalDate("2026-05-23T07:00:00+09:00", "en")).toBe("May 23, 2026");
+    expect(formatJournalDate("2026-10-04T23:30:00+09:00", "ko")).toBe("2026년 10월 4일");
+  });
+
+  it("gives every post the day written in its frontmatter", () => {
+    vi.stubEnv("TZ", "UTC");
+    const wrong: string[] = [];
+
+    for (const lang of ["ko", "en"] satisfies JournalLang[]) {
+      for (const post of getAllJournalPosts({ lang, includeDrafts: true })) {
+        const day = post.date.match(/^(\d{4})-(\d{2})-(\d{2})T[^+]*\+09:00$/);
+        if (!day) continue;
+        const want = formatJournalDate(`${day[1]}-${day[2]}-${day[3]}T12:00:00+09:00`, lang);
+        const shown = formatJournalDate(post.date, lang);
+        if (shown !== want) wrong.push(`${post.slug} (${lang}): ${shown}, written ${want}`);
+      }
+    }
+
+    expect(wrong).toEqual([]);
+  });
+});
diff --git a/web/src/lib/journal.ts b/web/src/lib/journal.ts
index 191c088c1..6f0ec59e1 100644
--- a/web/src/lib/journal.ts
+++ b/web/src/lib/journal.ts
@@ -128,10 +128,13 @@ export function getAdjacentJournalPosts(slug: string, lang: JournalLang = "ko")
   };
 }
 
+// The dates are written in Seoul (+09:00). Without a zone the day is the build
+// machine's, and a post written at 01:30 in Seoul is the day before in UTC.
 export function formatJournalDate(date: string, lang: JournalLang = "ko") {
   return new Intl.DateTimeFormat(lang === "ko" ? "ko-KR" : "en", {
     month: "long",
     day: "numeric",
     year: "numeric",
+    timeZone: "Asia/Seoul",
   }).format(new Date(date));
 }