fix: Infinite loop in timezones on the negative side of UTC (#12063)

* fix: Infinite loop in timezones on the negative side of UTC

* Update packages/features/calendars/lib/getAvailableDatesInMonth.test.ts

* Revert back to real system time after test

* Handle all dates as local time, given this all happens in the browser

.github/workflows/test.yml

@@ -15,3 +15,5 @@ jobs:
       - uses: ./.github/actions/yarn-install
       # Should be an 8GB machine as per https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners
       - run: yarn test
+      # We could add different timezones here that we need to run our tests in
+      - run: TZ=America/Los_Angeles yarn test -- --timeZoneDependentTestsOnly

packages/features/calendars/lib/getAvailableDatesInMonth.timezone.test.ts

@@ -1,4 +1,4 @@
-import { describe, expect, test } from "vitest";
+import { describe, expect, test, vi } from "vitest";
 
 import { getAvailableDatesInMonth } from "@calcom/features/calendars/lib/getAvailableDatesInMonth";
 import { daysInMonth, yyyymmdd } from "@calcom/lib/date-fns";
@@ -8,7 +8,7 @@ describe("Test Suite: Date Picker", () => {
     // *) Use right amount of days in given month. (28, 30, 31)
     test("it returns the right amount of days in a given month", () => {
       const currentDate = new Date();
-      const nextMonthDate = new Date(Date.UTC(currentDate.getFullYear(), currentDate.getMonth() + 1));
+      const nextMonthDate = new Date(currentDate.getFullYear(), currentDate.getMonth() + 1);
 
       const result = getAvailableDatesInMonth({
         browsingDate: nextMonthDate,
@@ -35,5 +35,33 @@ describe("Test Suite: Date Picker", () => {
 
       expect(result).toHaveLength(1);
     });
+
+    test("it translates correctly regardless of system time", () => {
+      {
+        // test a date in negative UTC offset
+        vi.useFakeTimers().setSystemTime(new Date("2023-10-24T13:27:00.000-07:00"));
+
+        const currentDate = new Date();
+        const result = getAvailableDatesInMonth({
+          browsingDate: currentDate,
+        });
+
+        expect(result).toHaveLength(daysInMonth(currentDate) - currentDate.getDate() + 1);
+      }
+      {
+        // test a date in positive UTC offset
+        vi.useFakeTimers().setSystemTime(new Date("2023-10-24T13:27:00.000+07:00"));
+
+        const currentDate = new Date();
+        const result = getAvailableDatesInMonth({
+          browsingDate: currentDate,
+        });
+
+        expect(result).toHaveLength(daysInMonth(currentDate) - currentDate.getDate() + 1);
+      }
+      // Undo the forced time we applied earlier, reset to system default.
+      vi.setSystemTime(vi.getRealSystemTime());
+      vi.useRealTimers();
+    });
   });
 });

packages/features/calendars/lib/getAvailableDatesInMonth.ts

@@ -5,7 +5,7 @@ import { daysInMonth, yyyymmdd } from "@calcom/lib/date-fns";
 // *) Dates in the past are not available.
 // *) Use right amount of days in given month. (28, 30, 31)
 export function getAvailableDatesInMonth({
-  browsingDate, // pass as UTC
+  browsingDate,
   minDate = new Date(),
   includedDates,
 }: {
@@ -15,12 +15,14 @@ export function getAvailableDatesInMonth({
 }) {
   const dates = [];
   const lastDateOfMonth = new Date(
-    Date.UTC(browsingDate.getFullYear(), browsingDate.getMonth(), daysInMonth(browsingDate))
+    browsingDate.getFullYear(),
+    browsingDate.getMonth(),
+    daysInMonth(browsingDate)
   );
   for (
     let date = browsingDate > minDate ? browsingDate : minDate;
     date <= lastDateOfMonth;
-    date = new Date(Date.UTC(date.getFullYear(), date.getMonth(), date.getDate() + 1))
+    date = new Date(date.getFullYear(), date.getMonth(), date.getDate() + 1)
   ) {
     // intersect included dates
     if (includedDates && !includedDates.includes(yyyymmdd(date))) {

vitest.workspace.ts

@@ -1,6 +1,13 @@
 import { defineWorkspace } from "vitest/config";
 
 const packagedEmbedTestsOnly = process.argv.includes("--packaged-embed-tests-only");
+const timeZoneDependentTestsOnly = process.argv.includes("--timeZoneDependentTestsOnly");
+// eslint-disable-next-line turbo/no-undeclared-env-vars
+const envTZ = process.env.TZ;
+if (timeZoneDependentTestsOnly && !envTZ) {
+  throw new Error("TZ environment variable is not set");
+}
+
 // defineWorkspace provides a nice type hinting DX
 const workspaces = packagedEmbedTestsOnly
   ? [
@@ -11,6 +18,19 @@ const workspaces = packagedEmbedTestsOnly
         },
       },
     ]
+  : // It doesn't seem to be possible to fake timezone per test, so we rerun the entire suite with different TZ. See https://github.com/vitest-dev/vitest/issues/1575#issuecomment-1439286286
+  timeZoneDependentTestsOnly
+  ? [
+      {
+        test: {
+          name: `TimezoneDependentTests:${envTZ}`,
+          include: ["packages/**/*.timezone.test.ts", "apps/**/*.timezone.test.ts"],
+          // TODO: Ignore the api until tests are fixed
+          exclude: ["**/node_modules/**/*", "packages/embeds/**/*"],
+          setupFiles: ["setupVitest.ts"],
+        },
+      },
+    ]
   : [
       {
         test: {
@@ -20,6 +40,7 @@ const workspaces = packagedEmbedTestsOnly
           setupFiles: ["setupVitest.ts"],
         },
       },
+
       {
         test: {
           name: "@calcom/closecom",