DEV: Add additional test for timezone switching during event creation

Test the case which has been resolved by core commit c9a6d9ac89

Co-authored-by: David Taylor <david@taylorhq.com>

test/javascripts/acceptance/post-event-builder-test.js

@@ -2,6 +2,7 @@ import { acceptance, query } from "discourse/tests/helpers/qunit-helpers";
 import { test } from "qunit";
 import { click, fillIn, visit } from "@ember/test-helpers";
 import selectKit from "discourse/tests/helpers/select-kit-helper";
+import sinon from "sinon";
 
 acceptance("Post event - composer", function (needs) {
   needs.user({ admin: true, can_create_discourse_post_event: true });
@@ -62,4 +63,55 @@ acceptance("Post event - composer", function (needs) {
       "bbcode is correct"
     );
   });
+
+  test("composer event builder - the timezone case", async function (assert) {
+    await visit("/");
+
+    // Freeze time
+    const newTimezone = "Europe/Paris";
+    const previousZone = moment.tz.guess();
+    const now = moment.tz("2022-04-04 23:15", newTimezone).valueOf();
+    sinon.useFakeTimers({
+      now,
+      toFake: ["Date"],
+      shouldAdvanceTime: true,
+      shouldClearNativeTimers: true,
+    });
+    sinon.stub(moment.tz, "guess");
+    moment.tz.guess.returns(newTimezone);
+    moment.tz.setDefault(newTimezone);
+
+    try {
+      await click("#create-topic");
+      const categoryChooser = selectKit(".category-chooser");
+      await categoryChooser.expand();
+      await categoryChooser.selectRowByValue(2);
+
+      await click(".toolbar-popup-menu-options .dropdown-select-box-header");
+      await click(".toolbar-popup-menu-options *[data-value='insertEvent']");
+
+      const modal = ".discourse-post-event-builder-modal";
+
+      // Select the date
+      await fillIn(`${modal} .from input[type=date]`, "2022-07-01");
+
+      // Select the timezone
+      const timezoneInput = selectKit(
+        `${modal} .event-field.timezone .timezone-input`
+      );
+      await timezoneInput.expand();
+      await timezoneInput.selectRowByValue("Europe/London");
+
+      // The date should be still the same?
+      assert.strictEqual(
+        query(`${modal} .from input[type=date]`).value,
+        "2022-07-01"
+      );
+    } finally {
+      // Unfreeze time
+      moment.tz.guess.returns(previousZone);
+      moment.tz.setDefault(previousZone);
+      sinon.restore();
+    }
+  });
 });