1 files changed, 66 insertions, 0 deletions
diff --git a/test/timezone.test.ts b/test/timezone.test.ts
new file mode 100644
index 0000000..f6c2230
--- /dev/null
+++ b/test/timezone.test.ts
@@ -0,0 +1,66 @@
+import { describe, expect, it } from "vitest";
+import { formatUtcOffset, isValidTimeZone, nextOffsetTransition, utcOffsetSeconds } from "../src/timezone.js";
+
+describe("isValidTimeZone", () => {
+  it("accepts IANA names", () => {
+    expect(isValidTimeZone("America/Chicago")).toBe(true);
+    expect(isValidTimeZone("Europe/Berlin")).toBe(true);
+    expect(isValidTimeZone("UTC")).toBe(true);
+  });
+
+  it("rejects invalid names", () => {
+    expect(isValidTimeZone("Central Time")).toBe(false);
+    expect(isValidTimeZone("")).toBe(false);
+    expect(isValidTimeZone("America/Springfield")).toBe(false);
+  });
+});
+
+describe("utcOffsetSeconds", () => {
+  it("returns CST offset in January", () => {
+    expect(utcOffsetSeconds("America/Chicago", new Date("2026-01-15T12:00:00Z"))).toBe(-6 * 3600);
+  });
+
+  it("returns CDT offset in July", () => {
+    expect(utcOffsetSeconds("America/Chicago", new Date("2026-07-15T12:00:00Z"))).toBe(-5 * 3600);
+  });
+
+  it("returns 0 for UTC", () => {
+    expect(utcOffsetSeconds("UTC", new Date("2026-07-15T12:00:00Z"))).toBe(0);
+  });
+
+  it("handles half-hour offsets", () => {
+    expect(utcOffsetSeconds("Asia/Kolkata", new Date("2026-07-15T12:00:00Z"))).toBe(5.5 * 3600);
+  });
+});
+
+describe("formatUtcOffset", () => {
+  it("formats negative, positive, and zero offsets", () => {
+    expect(formatUtcOffset(-6 * 3600)).toBe("UTC-06:00");
+    expect(formatUtcOffset(5.5 * 3600)).toBe("UTC+05:30");
+    expect(formatUtcOffset(0)).toBe("UTC+00:00");
+  });
+});
+
+describe("nextOffsetTransition", () => {
+  it("finds the exact spring-forward instant for Chicago", () => {
+    const transition = nextOffsetTransition("America/Chicago", new Date("2026-01-15T00:00:00Z"));
+    expect(transition).not.toBeNull();
+    // 2026-03-08 02:00 CST (UTC-6) -> 03:00 CDT (UTC-5): 08:00:00 UTC.
+    expect(transition!.at.toISOString()).toBe("2026-03-08T08:00:00.000Z");
+    expect(transition!.offsetBeforeSeconds).toBe(-6 * 3600);
+    expect(transition!.offsetAfterSeconds).toBe(-5 * 3600);
+  });
+
+  it("finds the exact fall-back instant for Chicago", () => {
+    const transition = nextOffsetTransition("America/Chicago", new Date("2026-07-15T00:00:00Z"));
+    expect(transition).not.toBeNull();
+    // 2026-11-01 02:00 CDT (UTC-5) -> 01:00 CST (UTC-6): 07:00:00 UTC.
+    expect(transition!.at.toISOString()).toBe("2026-11-01T07:00:00.000Z");
+    expect(transition!.offsetBeforeSeconds).toBe(-5 * 3600);
+    expect(transition!.offsetAfterSeconds).toBe(-6 * 3600);
+  });
+
+  it("returns null for fixed-offset zones", () => {
+    expect(nextOffsetTransition("UTC", new Date("2026-01-15T00:00:00Z"))).toBeNull();
+  });
+});