Input fields don't emit (and thus round-trip) offset value when binding against DateTimeOffset fields #6648
Description
Was working on the ASP.NET Core App-building Workshop and discovered an interesting issue when model binding to properties of type DateTimeOffset, especially when the application is running on a machine with a different system locale (and thus default time zone) than the DateTimeOffset values being edited.
Consider a Tag Helper use like <input asp-for="StartTime" />, where StartTime is defined as public DateTimeOffset StartTime { get; set; } on the model. The rendered HTML for the input field will not include the offset value: <input class="form-control" type="datetime-local" id="StartTime" name="StartTime" value="2017-08-14T09:00:00.000" /> . When this is posted back, the model bound value will now have an offset of the machine's system locale, even if the field wasn't changed, resulting in a different value.
We need to consider what the correct behavior of the helpers should be here, and how it interacts with the HTML5 datetime-local field type. Or perhaps we need specific guidance (and maybe helpers/binders) for handling DateTimeOffset values.