Incorrectly labeled days of month

[approachableGeek]

Describe the bug
November 2020 has two 1st's

To Reproduce
Steps to reproduce the behavior:

1. Clone master
2. Flutter pub get
3. Flutter run (example)
4. Select any calendar method
5. Scroll to November 2020
6. Sunday is correctly labeled as the 1st, Monday the 2nd is incorrectly labeled as the 1st as well shifting the entire month off my 1 day

Expected behavior
Days of the month should never repeat within the same month.

Screenshots

Smartphone:

• Device: iPhone11 Simulator
• OS: iOS13.5

Additional context
Flutter Channel master, 1.20.0-2.0.pre

[approachableGeek]

It appears that this line isn't doing what would be expected

Some-Calendar/lib/some_calendar_page.dart

Line 79 in 474ddbb

dateTime = dateTime..add(days: startDayOffset);

The new dateTime after adding a startDayOffset of 1 for the 1st of November is still November 1st and 23 hours
A not so great solution could be to add something like

var tempTime = dateTime;
dateTime = dateTime..add(days: startDayOffset);
if (dateTime.dateTime.day == tempTime.dateTime.day) {
   dateTime = dateTime..add(hours: 1);
}

I hope that this helps in finding the cause of the issue and it can come to a good solution.

[approachableGeek]

Looks like it is an issue with Jiffy from what I can tell anything between UTC-9:00 and UTC-2:00 is exhibiting this issue

import 'package:jiffy/jiffy.dart';
import 'package:timezone/timezone.dart' as tz;
import 'package:timezone/data/latest.dart' as tz;
import 'package:timezone/timezone.dart';

void main() {
  var incorrectOffsets = Set<int>();
  tz.initializeTimeZones();
  var locations = tz.timeZoneDatabase.locations;
  locations.values.forEach((location) {
    // MANIPULATING DATES
    var dateTime = Jiffy(TZDateTime(location, 2020, 11, 1));
    var nextDateTime = dateTime..add(days: 1);
    if (nextDateTime.day == 1) {
      incorrectOffsets.add(location.currentTimeZone.offset / 1000 / 60 ~/ 60);
    }
  });

  var inCorrectOffsetsList = incorrectOffsets.toList()..sort();
  inCorrectOffsetsList.forEach((offset) {
    print(offset);
  });
}