diff --git a/packages/compiler/src/frontend/lowering/lower-timers.ts b/packages/compiler/src/frontend/lowering/lower-timers.ts
index 03c9c5c1f..c76b2e667 100644
--- a/packages/compiler/src/frontend/lowering/lower-timers.ts
+++ b/packages/compiler/src/frontend/lowering/lower-timers.ts
@@ -15,6 +15,7 @@ import {
   typeKey,
 } from "../../ir/ir.js";
 import { buildFunctionAdapter } from "./coercions/builders.js";
+import { lowerOptionalNumber } from "./lower-exprs.js";
 import type { Lowerer } from "./lowerer.js";
 
 /** setTimeout invokes its callback with NO arguments, but @types/node's
@@ -409,6 +410,13 @@ function lowerTimerCall(
         type: F64,
         loc,
       };
+    } else if (ms.type.kind === "union") {
+      // A delay that may be undefined: an optional `ms?: number` passed on,
+      // or an element read (`delays[0]`) whose storage keeps the
+      // out-of-range undefined. Node converts the delay with ToNumber
+      // (`after *= 1`), so undefined is NaN, the omitted delay. Other
+      // unions stay as they are, and the lib boundary refuses them.
+      ms = lowerOptionalNumber(lowerer, ms, loc, expr.arguments.nodes[1]);
     }
     if (member === "setInterval") {
       return { kind: "libCall", fn: "timers.setInterval", args: [cb, ms], type: F64, loc };
diff --git a/packages/compiler/test/timer-delay-lowering.test.ts b/packages/compiler/test/timer-delay-lowering.test.ts
new file mode 100644
index 000000000..e4219c3c2
--- /dev/null
+++ b/packages/compiler/test/timer-delay-lowering.test.ts
@@ -0,0 +1,79 @@
+import { mkdtemp, readFile, rm, writeFile } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { expect, test } from "vitest";
+import { compile, deserializeModule } from "../src/index.js";
+import type { IrExpr } from "../src/ir/ir.js";
+
+/** The delay argument of every setTimeout/setInterval libCall in the module. */
+async function timerDelays(source: string): Promise<IrExpr[]> {
+  const dir = await mkdtemp(join(tmpdir(), "scriptc-timer-delays-"));
+  try {
+    const entry = join(dir, "main.ts");
+    await writeFile(entry, source);
+    const outPath = join(dir, "main.ir.json");
+    const result = await compile(entry, { outDir: dir, outPath, outputKind: "ir" });
+    if (!result.ok)
+      throw new Error(result.diagnostics.map((d) => `${d.code}: ${d.message}`).join("\n"));
+    const mod = deserializeModule(await readFile(outPath, "utf8"));
+    const delays: IrExpr[] = [];
+    const visit = (value: unknown): void => {
+      if (Array.isArray(value)) {
+        for (const item of value) visit(item);
+        return;
+      }
+      if (value === null || typeof value !== "object") return;
+      const node = value as { kind?: unknown; fn?: unknown; args?: IrExpr[] };
+      if (
+        node.kind === "libCall" &&
+        typeof node.fn === "string" &&
+        /^timers\.set(Timeout|TimeoutHandle|Interval)$/.test(node.fn)
+      )
+        delays.push(node.args![1]!);
+      for (const child of Object.values(value)) visit(child);
+    };
+    visit(mod.functions);
+    return delays;
+  } finally {
+    await rm(dir, { recursive: true, force: true });
+  }
+}
+
+test("number delays pass to the timer unchanged", async () => {
+  const delays = await timerDelays(
+    `function arm(ms: number): void {
+      setTimeout(() => console.log("a"), ms);
+      setTimeout(() => console.log("b"), 5);
+      const id = setInterval(() => clearInterval(id), ms);
+    }
+    arm(1);
+    export {};
+    `,
+  );
+  expect(delays.map((delay) => delay.kind)).toEqual(["varRef", "numLit", "varRef"]);
+  for (const delay of delays) expect(delay.type).toEqual({ kind: "f64" });
+});
+
+test("a number | undefined delay converts with ToNumber", async () => {
+  const delays = await timerDelays(
+    `interface Options { ms?: number }
+    function arm(options: Options): void {
+      setTimeout(() => console.log("a"), options.ms);
+      const id = setInterval(() => clearInterval(id), options.ms);
+    }
+    arm({});
+    export {};
+    `,
+  );
+  expect(delays).toHaveLength(2);
+  for (const delay of delays) {
+    // undefined is NaN, the omitted delay; a number passes through.
+    expect(delay).toMatchObject({
+      kind: "ternary",
+      type: { kind: "f64" },
+      cond: { kind: "unionIsTag" },
+      then: { kind: "bin", op: "/" },
+      else_: { kind: "unionNarrow", type: { kind: "f64" } },
+    });
+  }
+});
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/timer-optional-delays.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/timer-optional-delays.ts.json
new file mode 100644
index 000000000..f95051960
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/timer-optional-delays.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/timer-optional-delays.ts"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/timer-optional-delays.ts b/tests/corpus/timer-optional-delays.ts
new file mode 100644
index 000000000..788d6a193
--- /dev/null
+++ b/tests/corpus/timer-optional-delays.ts
@@ -0,0 +1,120 @@
+// setTimeout and setInterval delays typed `number | undefined`: an optional
+// `ms?: number` passed on, a number[] element read (its storage keeps the
+// out-of-range undefined), and a read past the end. Node converts the delay
+// with ToNumber, so undefined is NaN and the timer waits the default 1ms,
+// like an omitted delay. Timers sharing the 1ms bucket fire in registration
+// order; the other timers report through a counter so wall-clock timing
+// never decides the output.
+
+interface RefreshOptions {
+  label: string;
+  timeoutMs?: number;
+}
+
+const DELAYS = [1, 5];
+const firstBucket: string[] = [];
+const events: string[] = [];
+let pending = 0;
+
+function track(): () => void {
+  pending++;
+  return () => {
+    pending--;
+    if (pending === 0) {
+      console.log("1ms bucket:", firstBucket.join(", "));
+      console.log("events:", events.sort().join(", "));
+    }
+  };
+}
+
+// An optional field passed on, absent and present; the handle form clears.
+function schedule(options: RefreshOptions): ReturnType<typeof setTimeout> {
+  const done = track();
+  return setTimeout(() => {
+    firstBucket.push(options.label);
+    done();
+  }, options.timeoutMs);
+}
+schedule({ label: "absent" });
+schedule({ label: "one", timeoutMs: 1 });
+
+// Element reads in statement position: in range, and past the end.
+const doneFirst = track();
+setTimeout(() => {
+  firstBucket.push("DELAYS[0]");
+  doneFirst();
+}, DELAYS[0]);
+const doneMissing = track();
+setTimeout(() => {
+  firstBucket.push("DELAYS[9]");
+  doneMissing();
+}, DELAYS[9]);
+
+const cancelled = schedule({ label: "cancelled", timeoutMs: DELAYS[1] });
+clearTimeout(cancelled);
+pending--;
+
+// A binding initialized from an element read.
+const retry = DELAYS[1];
+const doneRetry = track();
+setTimeout(() => {
+  events.push(`retry after ${retry}`);
+  doneRetry();
+}, retry);
+
+// A guard that narrows the controller, not the delay.
+function withTimeout(ms?: number): void {
+  const controller = ms !== undefined ? new AbortController() : undefined;
+  const done = controller ? track() : undefined;
+  const timer = controller
+    ? setTimeout(() => {
+        controller.abort();
+        events.push(`aborted ${controller.signal.aborted}`);
+        done?.();
+      }, ms)
+    : undefined;
+  if (timer === undefined) events.push("no timer");
+}
+withTimeout();
+withTimeout(3);
+
+// An interval with an optional delay, cleared through its handle.
+function tickTwice(label: string, ms?: number): void {
+  const done = track();
+  let ticks = 0;
+  const id = setInterval(() => {
+    ticks++;
+    if (ticks === 2) {
+      clearInterval(id);
+      events.push(`${label} ticked ${ticks}`);
+      done();
+    }
+  }, ms);
+}
+tickTwice("default interval");
+tickTwice("element interval", DELAYS[0]);
+
+// The sleep idiom with an element delay.
+async function sleepThenLog(): Promise<void> {
+  const done = track();
+  await new Promise((resolve) => setTimeout(resolve, DELAYS[0]));
+  events.push("slept");
+  done();
+}
+void sleepThenLog();
+
+// A narrowed delay keeps the plain number path.
+function narrowed(ms: number | undefined): void {
+  if (ms === undefined) return;
+  const done = track();
+  setTimeout(() => {
+    events.push(`narrowed ${ms + 1}`);
+    done();
+  }, ms);
+}
+narrowed(2);
+narrowed(undefined);
+
+console.log("scheduled", pending);
+
+export {};
diff --git a/tests/harness/corpus-costs/timer-optional-delays.ts.json b/tests/harness/corpus-costs/timer-optional-delays.ts.json
new file mode 100644
index 000000000..fa2c86926
--- /dev/null
+++ b/tests/harness/corpus-costs/timer-optional-delays.ts.json
@@ -0,0 +1 @@
+{ "plain": 4000, "san": 73500 }