UWP background task give errors on List DateTime and IComparable

Hi all you knowledgeable developers,
I followed the steps for creating an out-of-process background task for the Xamarin Forms UWP portion of my app here:
https://docs.microsoft.com/en-us/windows/uwp/launch-resume/create-and-register-a-background-task
and I can do a "Hello world" call to the Background.

Now I want to actually do something and came up against some nasty compile errors of which here are a couple:

This line:
public DateTime LastLoginDate { get; set; }
gives this error:
Method 'MyApp.Models.UserShort.LastLoginDate.get()' returns 'System.DateTime', which is not a valid Windows Runtime type. Methods exposed to Windows Runtime must return only Windows Runtime types.

And this line:
public class CountID : IComparable<CountID>
gives this error:
Type 'BGActivities.Models.CountID' implements interface 'System.IComparable<BGActivities.Models.CountID>', which is not a Windows Runtime interface. All interfaces that are implemented by exported types must be Windows Runtime interfaces.

How can I get get to use DateTime and IComparable?
Have I missed some step in setting up the Background task?
Would there be plugins that I have left out?

Thanks!