There seems to be a copy-paste error here. You are using Culture.Calendar.GetYear(_selectedDate.Value) as the fallback for FixDay. It should be Culture.Calendar.GetDayOfMonth(_selectedDate.Value).

                    FixDay ?? Culture.Calendar.GetDayOfMonth(_selectedDate.Value),

The DateTime constructor used here is missing the Calendar argument. Without it, the constructor will default to the Gregorian calendar, which will cause issues with non-Gregorian cultures. You should use the overload that accepts a Calendar object.

                    a = new DateTime(Culture.Calendar.GetYear(a.Value), Culture.Calendar.GetMonth(a.Value), 1, Culture.Calendar);

This condition _picker_month is { Year: 1, Month: 1 } uses property patterns on DateTime, which relies on the Gregorian calendar. This will lead to incorrect behavior for non-Gregorian calendars. You've corrected this pattern in other places, but this one seems to have been missed. It should be updated to use Culture.Calendar to get the year and month.

            if (_picker_month.HasValue && Culture.Calendar.GetYear(_picker_month.Value) == 1 && Culture.Calendar.GetMonth(_picker_month.Value) == 1)

DateTime.TryParseExact is being called with CultureInfo.InvariantCulture, which forces the use of the Gregorian calendar for parsing. Since the year, month, and day variables are derived using Culture.Calendar, you should pass the Culture object to TryParseExact to ensure the correct calendar is used for parsing the date string.

            if (DateTime.TryParseExact($"{year}-{month}-{day}", "yyyy-M-d", Culture, DateTimeStyles.None, out var date))

In OnYearClickedAsync, when creating the new DateTime, there are two issues:

1. It's using Culture.Calendar.GetYear(_selectedDate.Value) instead of the year parameter passed to the method. The user's selected year should be used.
2. The DateTime constructor is missing the Calendar object, which will cause it to default to the Gregorian calendar.

Here's the corrected code.

                    new DateTime(
                        year,
                        Culture.Calendar.GetMonth(_selectedDate.Value),
                        Culture.Calendar.GetDayOfMonth(_selectedDate.Value),
                        Culture.Calendar.GetHour(_selectedDate.Value),
                        Culture.Calendar.GetMinute(_selectedDate.Value),
                        Culture.Calendar.GetSecond(_selectedDate.Value),
                        (int)Culture.Calendar.GetMilliseconds(_selectedDate.Value),
                        Culture.Calendar,
                        _selectedDate.Value.Kind)

This check uses direct DateTime properties (Year, Month) but should use calendar-aware methods for consistency with the rest of the PR. It should be: if (_picker_month.HasValue && Culture.Calendar.GetYear(_picker_month.Value) == 1 && Culture.Calendar.GetMonth(_picker_month.Value) == 1)

What does this change do, and it is required to be in the Culture's calendar?

this particular change doesn't actually do anything that I can tell but there are some of the Culture.Calendar methods that do make translations based on culture. This suggestion is to make everything use the same structure.

The indentation uses tabs instead of spaces, which is inconsistent with the file's formatting. The line should use consistent whitespace (spaces) to match the surrounding code.

The indentation uses tabs instead of spaces, which is inconsistent with the file's formatting. These lines should use consistent whitespace (spaces) to match the surrounding code.

This assignment to datePicker is useless, since its value is never read.