Bugfix/dst available hour mismatch (#4909)

* Need to use local time, not inviteeDate

* Use selected timeZone to perform localWorkingHours logic

* Adding comment

* More comments

packages/lib/slots.ts

@@ -42,7 +42,9 @@ const splitAvailableTime = (
 const getSlots = ({ inviteeDate, frequency, minimumBookingNotice, workingHours, eventLength }: GetSlots) => {
   // current date in invitee tz
   const startDate = dayjs().add(minimumBookingNotice, "minute");
-  const startOfDayUTC = dayjs.utc().startOf("day");
+  // This code is ran client side, startOf() does some conversions based on the
+  // local tz of the client. Sometimes this shifts the day incorrectly.
+  const startOfDayUTC = dayjs.utc().set("hour", 0).set("minute", 0).set("second", 0);
   const startOfInviteeDay = inviteeDate.startOf("day");
   // checks if the start date is in the past
 
@@ -61,9 +63,20 @@ const getSlots = ({ inviteeDate, frequency, minimumBookingNotice, workingHours,
     startTime: /* Why? */ startOfDayUTC.add(schedule.startTime, "minute"),
     endTime: /* Why? */ startOfDayUTC.add(schedule.endTime, "minute"),
   }));
-  const localWorkingHours = getWorkingHours({ utcOffset: -inviteeDate.utcOffset() }, workingHoursUTC).filter(
-    (hours) => hours.days.includes(inviteeDate.day())
-  );
+
+  // Dayjs does not expose the timeZone value publicly through .get("timeZone")
+  // instead, we as devs are required to somewhat hack our way to get the ...
+  // tz value as string
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  const timeZone: string = (inviteeDate as any)["$x"]["$timezone"];
+
+  const localWorkingHours = getWorkingHours(
+    {
+      // initialize current day with timeZone without conversion, just parse.
+      utcOffset: -dayjs.tz(dayjs(), timeZone).utcOffset(),
+    },
+    workingHoursUTC
+  ).filter((hours) => hours.days.includes(inviteeDate.day()));
 
   const slots: Dayjs[] = [];