Switch to DateOnly and TimeOnly Types in Pickers

[robloo]

Describe the feature you'd like

The DatePicker and TimePicker could be switched over to the new .NET 6 types for DateOnly and TimeOnly. Especially for the DatePicker, this could eliminate some potential issues with DateTimeOffset unexpectedly using certain time zone values.

As this would break existing apps and require .NET6, this should not be done for a few years. Perhaps around .NET 7/8 it would make sense to switch corresponding to the next major release of Avalonia that allows breaking changes.

Affected controls:

• DatePicker
• TimePicker
• Calendar
• CalendarDatePicker

Describe alternatives you've considered

1. Alternative is no changes.
2. Two selected value properties are provided: one for the old type, one for the new (idea discussed below)

Additional context

Date, Time, and Time Zone Enhancements in .NET 6

[maxkatz6]

As this would break existing apps and require .NET6

It won't break anything, if we add additional property for that. Moreover, next major release would be right time to do it, and not wait for .NET 7/8. Old property can be deprecated.
The problem is naming though.
It can be either this (no breaking change):

class TimePicker
{
   TimeSpan SelectedTime { get; set; }
#if NET6_OR_HIGHER
   TimeOnly SelectedTimeOnly { get; set; }
#endif
}

Or this (with breaking change, but easy workaround):

class TimePicker
{
#if NET6_OR_HIGHER
   TimeOnly SelectedTime { get; set; }
#endif
   TimeSpan SelectedTimeSpan { get; set; }
}

What is true that we definitely can't remove old TimeSpan/DateTime properties. It's pure pain, when two different targets of the same version have two API sets with same naming but different types.

[maxkatz6]

I personally would prefer new SelectedTimeOnly and SelectedDateOnly properties

[maxkatz6]

Though, on a second though, it would require quite a lot of API to duplicate. There also a DisplayDateStart, DisplayDateEnd and same for time picker. Not to mention internal logic should use new types where possible and fallback to old types on older targets.
Sadly, we can't really drop anything older than .NET 6 in near future as well to simplify it.

[robloo]

We dare not duplicate the API and create new properties. This needs to be a clean breaking changes that: 1. Simplifies usage (important) and 2. Prevents possible time zone errors 3. Modernizes types.

I was clear this might not be possible for some time. But when we are ready to set .net 6 as minimum it should be done.