diff --git a/server/src/services/routines-formatter-cache.test.ts b/server/src/services/routines-formatter-cache.test.ts
new file mode 100644
index 00000000..daf9064a
--- /dev/null
+++ b/server/src/services/routines-formatter-cache.test.ts
@@ -0,0 +1,51 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { nextCronTickInTimeZone } from "./routines.js";
+
+describe("nextCronTickInTimeZone formatter caching", () => {
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it("computes the next occurrence of a sparse monthly cron correctly", () => {
+    const next = nextCronTickInTimeZone(
+      "30 6 1 * *",
+      "America/New_York",
+      new Date("2026-06-02T00:00:00Z"),
+    );
+    expect(next).not.toBeNull();
+    // 06:30 America/New_York on July 1st is 10:30 UTC (EDT, UTC-4).
+    expect(next!.toISOString()).toBe("2026-07-01T10:30:00.000Z");
+  });
+
+  it("does not construct a new Intl.DateTimeFormat per minute-step", () => {
+    const RealDateTimeFormat = Intl.DateTimeFormat;
+    let constructions = 0;
+    // Must be a `function` (not an arrow) so the mock stays constructable
+    // when the code under test calls `new Intl.DateTimeFormat(...)`.
+    vi.spyOn(Intl, "DateTimeFormat").mockImplementation(function (
+      ...args: ConstructorParameters<typeof Intl.DateTimeFormat>
+    ) {
+      constructions += 1;
+      return new RealDateTimeFormat(...args);
+    } as unknown as typeof Intl.DateTimeFormat);
+
+    // A monthly cron scanned from just after the previous firing walks tens of
+    // thousands of minute-steps before it matches. Use a timezone that nothing
+    // else in this test file has warmed so cache misses are observable.
+    const next = nextCronTickInTimeZone(
+      "0 12 1 * *",
+      "Pacific/Kiritimati",
+      new Date("2026-06-01T12:01:00Z"),
+    );
+    expect(next).not.toBeNull();
+
+    // One construction to populate the per-timezone cache; the ~43k subsequent
+    // minute-steps must reuse it. Before the cache this was one construction
+    // per step, which blocked the event loop for minutes (#8033).
+    expect(constructions).toBeLessThanOrEqual(1);
+
+    constructions = 0;
+    nextCronTickInTimeZone("0 12 1 * *", "Pacific/Kiritimati", new Date("2026-06-01T12:01:00Z"));
+    expect(constructions).toBe(0);
+  });
+});
diff --git a/server/src/services/routines.ts b/server/src/services/routines.ts
index 7081377c..f7336368 100644
--- a/server/src/services/routines.ts
+++ b/server/src/services/routines.ts
@@ -90,7 +90,7 @@ function routineWebhookSecretConfigPath(secretId: string) {
 
 function assertTimeZone(timeZone: string) {
   try {
-    new Intl.DateTimeFormat("en-US", { timeZone }).format(new Date());
+    getZonedMinuteFormatter(timeZone).format(new Date());
   } catch {
     throw unprocessable(`Invalid timezone: ${timeZone}`);
   }
@@ -102,17 +102,33 @@ function floorToMinute(date: Date) {
   return copy;
 }
 
+// Constructing an Intl.DateTimeFormat costs ~1ms of ICU work, and
+// computeNextRun calls getZonedMinuteParts once per minute-step (up to
+// 366*24*60*5 iterations for sparse schedules), which can block the event
+// loop for minutes per scheduler tick. Formatter instances are immutable,
+// so cache one per timezone. See #8033.
+const zonedMinuteFormatterCache = new Map<string, Intl.DateTimeFormat>();
+
+function getZonedMinuteFormatter(timeZone: string) {
+  let formatter = zonedMinuteFormatterCache.get(timeZone);
+  if (!formatter) {
+    formatter = new Intl.DateTimeFormat("en-US", {
+      timeZone,
+      hour12: false,
+      year: "numeric",
+      month: "numeric",
+      day: "numeric",
+      hour: "numeric",
+      minute: "numeric",
+      weekday: "short",
+    });
+    zonedMinuteFormatterCache.set(timeZone, formatter);
+  }
+  return formatter;
+}
+
 function getZonedMinuteParts(date: Date, timeZone: string) {
-  const formatter = new Intl.DateTimeFormat("en-US", {
-    timeZone,
-    hour12: false,
-    year: "numeric",
-    month: "numeric",
-    day: "numeric",
-    hour: "numeric",
-    minute: "numeric",
-    weekday: "short",
-  });
+  const formatter = getZonedMinuteFormatter(timeZone);
   const parts = formatter.formatToParts(date);
   const map = Object.fromEntries(parts.map((part) => [part.type, part.value]));
   const weekday = WEEKDAY_INDEX[map.weekday ?? ""];
@@ -141,7 +157,7 @@ function matchesCronMinute(expression: string, timeZone: string, date: Date) {
   );
 }
 
-function nextCronTickInTimeZone(expression: string, timeZone: string, after: Date) {
+export function nextCronTickInTimeZone(expression: string, timeZone: string, after: Date) {
   const trimmed = expression.trim();
   assertTimeZone(timeZone);
   const error = validateCron(trimmed);