Make scheduling and dates timezone-safe

packages/domain/tests/domain.test.ts

@@ -3,14 +3,17 @@
 import { describe, expect, test } from "bun:test";
 import {
   formatZonedDateTime,
+  addZonedCalendarInterval,
   getDefaultScheduledSendAt,
   isValidTimeZone,
   toLocalDateTimeInputValue,
+  zonedDateTimeToInstant,
 } from "../src/time-zone";
 import {
   EXPENSE_CATEGORIES,
   formatElapsedSeconds,
   getEffectiveInvoiceStatus,
+  getDaysPastDue,
   parseReceiptText,
 } from "../src";
 
@@ -26,6 +29,17 @@ describe("shared domain behavior", () => {
     expect(getEffectiveInvoiceStatus("paid", yesterday)).toBe("paid");
   });
 
+  test("counts calendar days rather than 24-hour blocks across fall DST", () => {
+    expect(
+      getDaysPastDue(
+        "sent",
+        "2026-11-01",
+        "America/New_York",
+        new Date("2026-11-02T17:00:00.000Z"),
+      ),
+    ).toBe(1);
+  });
+
   test("formats elapsed time", () => {
     expect(formatElapsedSeconds(3_661)).toBe("01:01:01");
   });
@@ -56,6 +70,69 @@ describe("time-zone helpers", () => {
     expect(isValidTimeZone("not/a-zone")).toBe(false);
   });
 
+  test("converts Eastern wall time to the correct absolute instant", () => {
+    expect(
+      zonedDateTimeToInstant(
+        "2026-08-17T09:00",
+        "America/New_York",
+      ).toISOString(),
+    ).toBe("2026-08-17T13:00:00.000Z");
+  });
+
+  test("rejects nonexistent spring-forward wall times", () => {
+    expect(() =>
+      zonedDateTimeToInstant("2026-03-08T02:30", "America/New_York"),
+    ).toThrow("does not exist");
+  });
+
+  test("disambiguates both occurrences of a fall-back wall time", () => {
+    expect(
+      zonedDateTimeToInstant(
+        "2026-11-01T01:30",
+        "America/New_York",
+        "earlier",
+      ).toISOString(),
+    ).toBe("2026-11-01T05:30:00.000Z");
+    expect(
+      zonedDateTimeToInstant(
+        "2026-11-01T01:30",
+        "America/New_York",
+        "later",
+      ).toISOString(),
+    ).toBe("2026-11-01T06:30:00.000Z");
+  });
+
+  test("supports half-hour DST transitions", () => {
+    const earlier = zonedDateTimeToInstant(
+      "2026-04-05T01:45",
+      "Australia/Lord_Howe",
+      "earlier",
+    );
+    const later = zonedDateTimeToInstant(
+      "2026-04-05T01:45",
+      "Australia/Lord_Howe",
+      "later",
+    );
+    expect(later.getTime() - earlier.getTime()).toBe(30 * 60_000);
+  });
+
+  test("preserves Eastern wall time across DST and clamps month end", () => {
+    expect(
+      addZonedCalendarInterval(
+        new Date("2026-03-01T14:00:00.000Z"),
+        "weekly",
+        "America/New_York",
+      ).toISOString(),
+    ).toBe("2026-03-08T13:00:00.000Z");
+    expect(
+      addZonedCalendarInterval(
+        new Date("2026-01-31T14:00:00.000Z"),
+        "monthly",
+        "America/New_York",
+      ).toISOString(),
+    ).toBe("2026-02-28T14:00:00.000Z");
+  });
+
   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");