Why doesn't .NET use an unsigned integer type like uint(UInt32) or ushort (UInt16) for properties that cannot be negative, like DateTime.Year and DateTime.Month. In fact, I've never seen them used in the FCL to my recollection. When are we supposed to use them?
To quote "Language Independence and Language-Independent Components"
The .NET Framework is language independent. This means that, as a developer, you can develop in one of the many languages that target the .NET Framework, such as C#, C++/CLI, Eiffel, F#, IronPython, IronRuby, PowerBuilder, Visual Basic, Visual COBOL, and Windows PowerShell. You can access the types and members of class libraries developed for the .NET Framework without having to know the language in which they were originally written and without having to follow any of the original language's conventions. If you are a component developer, your component can be accessed by any .NET Framework app regardless of its language.
Not all languages support unsigned integers, so because of that using unsigned integers in public or protected methods makes your code non CLS compliant. You are allowed to use them on private and internal members, just not on public or protected ones. Because of this the .NET framework tries to make all of it's public methods and members CLS compliant so they can be used from any language.