Fix/DST issues (#7462)

* use the organizer timezone and invitee timezone to show slots

* type fixes

* set start date on selected TZ on booking payload

* add considerations when only the invitee is on DTS

* Apply suggestions from code review

Co-authored-by: alannnc <alannnc@gmail.com>
Co-authored-by: Omar López <zomars@me.com>

* fixes typo

* Apply suggestions from code review

Co-authored-by: alannnc <alannnc@gmail.com>

* cleaning

---------

Co-authored-by: Alex van Andel <me@alexvanandel.com>
Co-authored-by: Omar López <zomars@me.com>
Co-authored-by: alannnc <alannnc@gmail.com>

packages/features/bookings/lib/handleNewBooking.ts

@@ -19,7 +19,7 @@ import { cancelScheduledJobs, scheduleTrigger } from "@calcom/app-store/zapier/l
 import EventManager from "@calcom/core/EventManager";
 import { getEventName } from "@calcom/core/event";
 import { getUserAvailability } from "@calcom/core/getUserAvailability";
-import type { ConfigType } from "@calcom/dayjs";
+import type { ConfigType, Dayjs } from "@calcom/dayjs";
 import dayjs from "@calcom/dayjs";
 import {
   sendAttendeeRequestEmail,
@@ -35,6 +35,7 @@ import { deleteScheduledSMSReminder } from "@calcom/features/ee/workflows/lib/re
 import getWebhooks from "@calcom/features/webhooks/lib/getWebhooks";
 import { isPrismaObjOrUndefined, parseRecurringEvent } from "@calcom/lib";
 import { getVideoCallUrl } from "@calcom/lib/CalEventParser";
+import { getDSTDifference, isInDST } from "@calcom/lib/date-fns";
 import { getDefaultEvent, getGroupName, getUsernameList } from "@calcom/lib/defaultEvents";
 import { getErrorFromUnknown } from "@calcom/lib/errors";
 import getPaymentAppData from "@calcom/lib/getPaymentAppData";
@@ -111,16 +112,40 @@ const isWithinAvailableHours = (
   timeSlot: { start: ConfigType; end: ConfigType },
   {
     workingHours,
+    organizerTimeZone,
+    inviteeTimeZone,
   }: {
     workingHours: WorkingHours[];
+    organizerTimeZone: string;
+    inviteeTimeZone: string;
   }
 ) => {
   const timeSlotStart = dayjs(timeSlot.start).utc();
   const timeSlotEnd = dayjs(timeSlot.end).utc();
+  const isOrganizerInDST = isInDST(dayjs().tz(organizerTimeZone));
+  const isInviteeInDST = isInDST(dayjs().tz(organizerTimeZone));
+  const isOrganizerInDSTWhenSlotStart = isInDST(timeSlotStart.tz(organizerTimeZone));
+  const isInviteeInDSTWhenSlotStart = isInDST(timeSlotStart.tz(inviteeTimeZone));
+  const organizerDSTDifference = getDSTDifference(organizerTimeZone);
+  const inviteeDSTDifference = getDSTDifference(inviteeTimeZone);
+  const sameDSTUsers = isOrganizerInDSTWhenSlotStart === isInviteeInDSTWhenSlotStart;
+  const organizerDST = isOrganizerInDST === isOrganizerInDSTWhenSlotStart;
+  const inviteeDST = isInviteeInDST === isInviteeInDSTWhenSlotStart;
+  const getTime = (slotTime: Dayjs, minutes: number) =>
+    slotTime
+      .startOf("day")
+      .add(
+        sameDSTUsers && organizerDST && inviteeDST
+          ? minutes
+          : minutes -
+              (isOrganizerInDSTWhenSlotStart || isOrganizerInDST
+                ? organizerDSTDifference
+                : inviteeDSTDifference),
+        "minutes"
+      );
   for (const workingHour of workingHours) {
-    // TODO: Double check & possibly fix timezone conversions.
-    const startTime = timeSlotStart.startOf("day").add(workingHour.startTime, "minute");
-    const endTime = timeSlotEnd.startOf("day").add(workingHour.endTime, "minute");
+    const startTime = getTime(timeSlotStart, workingHour.startTime);
+    const endTime = getTime(timeSlotEnd, workingHour.endTime);
     if (
       workingHour.days.includes(timeSlotStart.day()) &&
       // UTC mode, should be performant.
@@ -247,7 +272,7 @@ async function ensureAvailableUsers(
   eventType: Awaited<ReturnType<typeof getEventTypesFromDB>> & {
     users: IsFixedAwareUser[];
   },
-  input: { dateFrom: string; dateTo: string },
+  input: { dateFrom: string; dateTo: string; timeZone: string },
   recurringDatesInfo?: {
     allRecurringDates: string[] | undefined;
     currentRecurringIndex: number | undefined;
@@ -271,6 +296,8 @@ async function ensureAvailableUsers(
         { start: input.dateFrom, end: input.dateTo },
         {
           workingHours,
+          organizerTimeZone: eventType.timeZone || eventType?.schedule?.timeZone || user.timeZone,
+          inviteeTimeZone: input.timeZone,
         }
       )
     ) {
@@ -567,6 +594,7 @@ async function handler(
       {
         dateFrom: reqBody.start,
         dateTo: reqBody.end,
+        timeZone: reqBody.timeZone,
       },
       {
         allRecurringDates,

packages/features/ee/teams/components/v2/TeamAvailabilityTimes.tsx

@@ -1,8 +1,8 @@
 import classNames from "classnames";
 import React from "react";
-import { ITimezone } from "react-timezone-select";
+import type { ITimezone } from "react-timezone-select";
 
-import { Dayjs } from "@calcom/dayjs";
+import type { Dayjs } from "@calcom/dayjs";
 import { useLocale } from "@calcom/lib/hooks/useLocale";
 import getSlots from "@calcom/lib/slots";
 import { trpc } from "@calcom/trpc/react";
@@ -42,6 +42,7 @@ export default function TeamAvailabilityTimes(props: Props) {
         workingHours: data?.workingHours || [],
         minimumBookingNotice: 0,
         eventLength: props.frequency,
+        organizerTimeZone: `${data?.timeZone}`,
       })
     : [];