Support setting a timezone with `process.env.TZ` and `Bun.env.TZ` (#3018)

* Support setting a timezone via `process.env.TZ`

* Implement `setTimeZone` in `bun:jsc` module

* [breaking] `bun:test` now defaults to `Etc/UTC` timezone

---------

Co-authored-by: Jarred Sumner <709451+Jarred-Sumner@users.noreply.github.com>

2 files changed, 52 insertions, 0 deletions

diff --git a/test/js/bun/jsc/bun-jsc.test.ts b/test/js/bun/jsc/bun-jsc.test.ts
index aa93ce90a..f16a6a781 100644
--- a/test/js/bun/jsc/bun-jsc.test.ts
+++ b/test/js/bun/jsc/bun-jsc.test.ts
@@ -22,6 +22,7 @@ import {
   reoptimizationRetryCount,
   drainMicrotasks,
   startRemoteDebugger,
+  setTimeZone,
 } from "bun:jsc";
 
 describe("bun:jsc", () => {
@@ -110,4 +111,33 @@ describe("bun:jsc", () => {
   it("getProtectedObjects", () => {
     expect(getProtectedObjects().length).toBeGreaterThan(0);
   });
+
+  it("setTimeZone", () => {
+    var origTimezone = Intl.DateTimeFormat().resolvedOptions().timeZone;
+    const realOrigTimezone = origTimezone;
+    if (origTimezone === "America/Anchorage") {
+      origTimezone = "America/New_York";
+    }
+    const origDate = new Date();
+    origDate.setSeconds(0);
+    origDate.setMilliseconds(0);
+    origDate.setMinutes(0);
+    const origDateString = origDate.toString();
+    expect(origTimezone).toBeDefined();
+    expect(origTimezone).not.toBe("America/Anchorage");
+    expect(setTimeZone("America/Anchorage")).toBe("America/Anchorage");
+    expect(Intl.DateTimeFormat().resolvedOptions().timeZone).toBe("America/Anchorage");
+    if (realOrigTimezone === origTimezone) {
+      const newDate = new Date();
+      newDate.setSeconds(0);
+      newDate.setMilliseconds(0);
+      newDate.setMinutes(0);
+      const newDateString = newDate.toString();
+      expect(newDateString).not.toBe(origDateString);
+    }
+
+    expect(setTimeZone(realOrigTimezone)).toBe(realOrigTimezone);
+
+    expect(Intl.DateTimeFormat().resolvedOptions().timeZone).toBe(origTimezone);
+  });
 });
diff --git a/test/js/node/process/process.test.js b/test/js/node/process/process.test.js
index 414392a79..c9f92362c 100644
--- a/test/js/node/process/process.test.js
+++ b/test/js/node/process/process.test.js
@@ -98,6 +98,28 @@ it("process.env is spreadable and editable", () => {
   expect(eval(`globalThis.process.env.USER = "${orig}"`)).toBe(orig);
 });
 
+it("process.env.TZ", () => {
+  var origTimezone = Intl.DateTimeFormat().resolvedOptions().timeZone;
+
+  // the default timezone is Etc/UTC
+  if (!"TZ" in process.env) {
+    expect(origTimezone).toBe("Etc/UTC");
+  }
+
+  const realOrigTimezone = origTimezone;
+  if (origTimezone === "America/Anchorage") {
+    origTimezone = "America/New_York";
+  }
+
+  const target = "America/Anchorage";
+  const tzKey = String("TZ" + " ").substring(0, 2);
+  process.env[tzKey] = target;
+  expect(process.env[tzKey]).toBe(target);
+  expect(Intl.DateTimeFormat().resolvedOptions().timeZone).toBe(target);
+  process.env[tzKey] = origTimezone;
+  expect(Intl.DateTimeFormat().resolvedOptions().timeZone).toBe(realOrigTimezone);
+});
+
 it("process.version starts with v", () => {
   expect(process.version.startsWith("v")).toBeTruthy();
 });