fix: missing last day of month

## What does this PR do?

- Fix that the last day in Oktober was always shown as unavailable
This happened when availability was in Europe/London timezone

- Fix that the last day of November was always shown as unavailable
This happened also for other timezones

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

## How should this be tested?

I recommend testing it in staging instead of local, as I experienced some different behaviors on local 

- Reproduce the 2 bugs as described before my fix
- If you were able to reproduce it, see if it is fixed with my fixes in this PR (last day of octber and November should be shown as available)
- Check if availability starts and ends at the correct time on days were DST happens

packages/features/calendars/DatePicker.tsx

@@ -139,7 +139,10 @@ const Days = ({
     return dates;
   };
 
-  const includedDates = currentDate.isSame(browsingDate, "month")
+  const utcBrowsingDateWithOffset = browsingDate.utc().add(browsingDate.utcOffset(), "minute");
+  const utcCurrentDateWithOffset = currentDate.utc().add(browsingDate.utcOffset(), "minute");
+
+  const includedDates = utcCurrentDateWithOffset.isSame(utcBrowsingDateWithOffset, "month")
     ? availableDates(props.includedDates)
     : props.includedDates;
 

packages/lib/date-ranges.test.ts

@@ -29,6 +29,25 @@ describe("processWorkingHours", () => {
       end: dayjs(`${dateTo.tz(timeZone).format("YYYY-MM-DD")}T21:00:00Z`).tz(timeZone),
     });
   });
+  it("should have availability on last day of month in the month were DST starts", () => {
+    const item = {
+      days: [0, 1, 2, 3, 4, 5, 6], // Monday to Sunday
+      startTime: new Date(Date.UTC(2023, 5, 12, 8, 0)), // 8 AM
+      endTime: new Date(Date.UTC(2023, 5, 12, 17, 0)), // 5 PM
+    };
+
+    const timeZone = "Europe/London";
+
+    const dateFrom = dayjs().month(9).date(24); // starts before DST change
+    const dateTo = dayjs().startOf("day").month(10).date(1); // first day of November
+
+    const results = processWorkingHours({ item, timeZone, dateFrom, dateTo });
+
+    const lastAvailableSlot = results[results.length - 1];
+
+    expect(lastAvailableSlot.start.date()).toBe(31);
+  });
+
   it("should return the correct working hours in the month were DST ends", () => {
     const item = {
       days: [0, 1, 2, 3, 4, 5, 6], // Monday to Sunday

packages/lib/date-ranges.ts

@@ -23,15 +23,20 @@ export function processWorkingHours({
 }) {
   const results = [];
   for (let date = dateFrom.tz(timeZone).startOf("day"); dateTo.isAfter(date); date = date.add(1, "day")) {
-    const dateInTz = date.tz(timeZone);
+    const fromOffset = dateFrom.tz(timeZone).utcOffset();
+    const offset = date.tz(timeZone).utcOffset();
 
+    // it always has to be start of the day (midnight) even when DST changes
+    const dateInTz = date.add(fromOffset - offset, "minutes").tz(timeZone);
     if (!item.days.includes(dateInTz.day())) {
       continue;
     }
 
-    let start = dateInTz.hour(item.startTime.getUTCHours()).minute(item.startTime.getUTCMinutes()).second(0);
+    let start = dateInTz
+      .add(item.startTime.getUTCHours(), "hours")
+      .add(item.startTime.getUTCMinutes(), "minutes");
 
-    let end = dateInTz.hour(item.endTime.getUTCHours()).minute(item.endTime.getUTCMinutes()).second(0);
+    let end = dateInTz.add(item.endTime.getUTCHours(), "hours").add(item.endTime.getUTCMinutes(), "minutes");
 
     const offsetBeginningOfDay = dayjs(start.format("YYYY-MM-DD hh:mm")).tz(timeZone).utcOffset();
     const offsetDiff = start.utcOffset() - offsetBeginningOfDay; // there will be 60 min offset on the day day of DST change