Add scheduled invoice delivery

packages/domain/tests/domain.test.ts

@@ -1,6 +1,12 @@
 /// <reference types="bun" />
 
 import { describe, expect, test } from "bun:test";
+import {
+  formatZonedDateTime,
+  getDefaultScheduledSendAt,
+  isValidTimeZone,
+  toLocalDateTimeInputValue,
+} from "../src/time-zone";
 import {
   EXPENSE_CATEGORIES,
   formatElapsedSeconds,
@@ -33,3 +39,25 @@ describe("shared domain behavior", () => {
     expect(receipt.items[0]?.name).toBe("Coffee");
   });
 });
+
+describe("time-zone helpers", () => {
+  test("formats the same instant in the selected IANA time zone", () => {
+    const instant = new Date("2026-08-17T16:30:00.000Z");
+    expect(formatZonedDateTime(instant, "America/New_York")).toContain(
+      "12:30 PM",
+    );
+    expect(formatZonedDateTime(instant, "America/Los_Angeles")).toContain(
+      "9:30 AM",
+    );
+  });
+
+  test("validates IANA time zones", () => {
+    expect(isValidTimeZone("America/New_York")).toBe(true);
+    expect(isValidTimeZone("not/a-zone")).toBe(false);
+  });
+
+  test("rounds the default schedule to the next local hour", () => {
+    const result = getDefaultScheduledSendAt(new Date(2026, 7, 17, 10, 42, 19));
+    expect(toLocalDateTimeInputValue(result)).toBe("2026-08-17T11:00");
+  });
+});