From dd0c2dbff226f5f72962106a2b13509a6e6ac0d2 Mon Sep 17 00:00:00 2001
From: Alex van Andel <me@alexvanandel.com>
Date: Fri, 3 Nov 2023 03:13:42 +0000
Subject: [PATCH] fix: Set the dateFrom to startOf day otherwise bad fromOffset
 is used (#12210)

---
 packages/lib/date-ranges.test.ts | 102 ++++++++++++++++++++++++++++++-
 packages/lib/date-ranges.ts      |   2 +-
 2 files changed, 102 insertions(+), 2 deletions(-)

diff --git a/packages/lib/date-ranges.test.ts b/packages/lib/date-ranges.test.ts
index 55f8f8b721..4a3769b378 100644
--- a/packages/lib/date-ranges.test.ts
+++ b/packages/lib/date-ranges.test.ts
@@ -1,4 +1,4 @@
-import { describe, expect, it } from "vitest";
+import { describe, expect, it, vi } from "vitest";
 
 import dayjs from "@calcom/dayjs";
 
@@ -48,6 +48,106 @@ describe("processWorkingHours", () => {
 
     expect(lastAvailableSlot.start.date()).toBe(31);
   });
+
+  it("It has the correct working hours on date of DST change (- tz)", () => {
+    vi.useFakeTimers().setSystemTime(new Date("2023-11-05T13:26:14.000Z"));
+
+    const item = {
+      days: [1, 2, 3, 4, 5],
+      startTime: new Date(Date.UTC(2023, 5, 12, 9, 0)), // 9 AM
+      endTime: new Date(Date.UTC(2023, 5, 12, 17, 0)), // 5 PM
+    };
+
+    const timeZone = "America/New_York";
+
+    const dateFrom = dayjs();
+    const dateTo = dayjs().endOf("month");
+
+    const results = processWorkingHours({ item, timeZone, dateFrom, dateTo });
+
+    expect(results).toStrictEqual([
+      {
+        start: dayjs("2023-11-06T14:00:00.000Z").tz(timeZone),
+        end: dayjs("2023-11-06T22:00:00.000Z").tz(timeZone),
+      },
+      {
+        start: dayjs("2023-11-07T14:00:00.000Z").tz(timeZone),
+        end: dayjs("2023-11-07T22:00:00.000Z").tz(timeZone),
+      },
+      {
+        start: dayjs("2023-11-08T14:00:00.000Z").tz(timeZone),
+        end: dayjs("2023-11-08T22:00:00.000Z").tz(timeZone),
+      },
+      {
+        start: dayjs("2023-11-09T14:00:00.000Z").tz(timeZone),
+        end: dayjs("2023-11-09T22:00:00.000Z").tz(timeZone),
+      },
+      {
+        start: dayjs("2023-11-10T14:00:00.000Z").tz(timeZone),
+        end: dayjs("2023-11-10T22:00:00.000Z").tz(timeZone),
+      },
+      {
+        start: dayjs("2023-11-13T14:00:00.000Z").tz(timeZone),
+        end: dayjs("2023-11-13T22:00:00.000Z").tz(timeZone),
+      },
+      {
+        start: dayjs("2023-11-14T14:00:00.000Z").tz(timeZone),
+        end: dayjs("2023-11-14T22:00:00.000Z").tz(timeZone),
+      },
+      {
+        start: dayjs("2023-11-15T14:00:00.000Z").tz(timeZone),
+        end: dayjs("2023-11-15T22:00:00.000Z").tz(timeZone),
+      },
+      {
+        start: dayjs("2023-11-16T14:00:00.000Z").tz(timeZone),
+        end: dayjs("2023-11-16T22:00:00.000Z").tz(timeZone),
+      },
+      {
+        start: dayjs("2023-11-17T14:00:00.000Z").tz(timeZone),
+        end: dayjs("2023-11-17T22:00:00.000Z").tz(timeZone),
+      },
+      {
+        start: dayjs("2023-11-20T14:00:00.000Z").tz(timeZone),
+        end: dayjs("2023-11-20T22:00:00.000Z").tz(timeZone),
+      },
+      {
+        start: dayjs("2023-11-21T14:00:00.000Z").tz(timeZone),
+        end: dayjs("2023-11-21T22:00:00.000Z").tz(timeZone),
+      },
+      {
+        start: dayjs("2023-11-22T14:00:00.000Z").tz(timeZone),
+        end: dayjs("2023-11-22T22:00:00.000Z").tz(timeZone),
+      },
+      {
+        start: dayjs("2023-11-23T14:00:00.000Z").tz(timeZone),
+        end: dayjs("2023-11-23T22:00:00.000Z").tz(timeZone),
+      },
+      {
+        start: dayjs("2023-11-24T14:00:00.000Z").tz(timeZone),
+        end: dayjs("2023-11-24T22:00:00.000Z").tz(timeZone),
+      },
+      {
+        start: dayjs("2023-11-27T14:00:00.000Z").tz(timeZone),
+        end: dayjs("2023-11-27T22:00:00.000Z").tz(timeZone),
+      },
+      {
+        start: dayjs("2023-11-28T14:00:00.000Z").tz(timeZone),
+        end: dayjs("2023-11-28T22:00:00.000Z").tz(timeZone),
+      },
+      {
+        start: dayjs("2023-11-29T14:00:00.000Z").tz(timeZone),
+        end: dayjs("2023-11-29T22:00:00.000Z").tz(timeZone),
+      },
+      {
+        start: dayjs("2023-11-30T14:00:00.000Z").tz(timeZone),
+        end: dayjs("2023-11-30T22:00:00.000Z").tz(timeZone),
+      },
+    ]);
+
+    vi.setSystemTime(vi.getRealSystemTime());
+    vi.useRealTimers();
+  });
+
   // TEMPORAIRLY SKIPPING THIS TEST - Started failing after 29th Oct
   it.skip("should return the correct working hours in the month were DST ends", () => {
     const item = {
diff --git a/packages/lib/date-ranges.ts b/packages/lib/date-ranges.ts
index 324f82dd08..fcef54a649 100644
--- a/packages/lib/date-ranges.ts
+++ b/packages/lib/date-ranges.ts
@@ -23,7 +23,7 @@ export function processWorkingHours({
 }) {
   const results = [];
   for (let date = dateFrom.tz(timeZone).startOf("day"); dateTo.isAfter(date); date = date.add(1, "day")) {
-    const fromOffset = dateFrom.tz(timeZone).utcOffset();
+    const fromOffset = dateFrom.tz(timeZone).startOf("day").utcOffset();
     const offset = date.tz(timeZone).utcOffset();
 
     // it always has to be start of the day (midnight) even when DST changes