Do I need to mock .net libraries?

Completely agree about the static nature. Static items are painful in unit tests. Which is why, as you suggest, I have an interface for time values which can be mocked in unit tests. More info https://adamstorr.azurewebsites.net/blog/mocking-the-clock

I don't understand the sentiment here. Many classes can't be properly unit tested without dependency inversion, and if you are applying the dependency inversion pattern to enable testing, your options are to write concrete implementations that exhibit the behavior you want to test, or use mocks. Mocks are objectively faster and more maintainable than writing concrete implementations.

Can elaborate on 'waste a lot of your time'? Or are you suggesting that unit testing is a waste of time?

You don't need to test the wrapper class as long as you make it a simple passthrough. You use the interface to make the consuming code testable, but the wrapper can just be marked with [IgnoreDuringTestCoverage] since you can rely on the assumption that the framework code is tested/works reliably.

sometimes you want to test how your SUT (system under test) behaves given the output of a 3rd party.

so for /u/NetFutility's benefit, here's a trivial example: e.g., my SUT greets you depending on the time of day. My SUT should return 1 of 3 greetings: "Good morning", "Good afternoon", or "Good evening"

My SUT could have easily determined the greeting like this:

if (DateTime.Now.Hour < 12)
{
    return "Good morning";
}

But that's not easily testable unless you run your tests at different times of the day. So we refactor the SUT to accept a provider that returns the current datetime.

public class Greeter
{
    private readonly IDateTimeProvider _dateTimeProvider = null;
    public Greeter(IDateTimeProvider dateTimeProvider)
    {
        _dateTimeProvider = dateTimeProvider;
    }
    public string Greet()
    {
        var hour = _dateTimeProvider.Now.Hour;

        if (hour < 12)
            return "Good morning";

        if (hour < 18)
            return "Good afternoon";

        return "Good evening";
    }
}

Here's the interface and default implementation that uses DateTime.Now:

public interface IDateTimeProvider
{
    public DateTime Now { get; }
}
public class DefaultDateTimeProvider : IDateTimeProvider
{
    public DateTime Now => DateTime.Now;
}

Then, you just need 3 tests (using xUnit and Moq) for 100% code coverage:

[Fact]
public void GreeterTest_Morning()
{
    Mock<IDateTimeProvider> mockDtp = new Mock<IDateTimeProvider>();
    mockDtp.SetupGet(it => it.Now).Returns(new DateTime(1, 1, 1, 8, 0, 0)); //8:00 a.m.
    string expectedGreeting = "Good morning";

    Greeter greeter = new Greeter(mockDtp.Object);
    string actualGreeting = greeter.Greet();

    Assert.Equal(expectedGreeting, actualGreeting);
}

[Fact]
public void GreeterTest_Afternoon()
{
    Mock<IDateTimeProvider> mockDtp = new Mock<IDateTimeProvider>();
    mockDtp.SetupGet(it => it.Now).Returns(new DateTime(1, 1, 1, 14, 0, 0)); //2:00 p.m.
    string expectedGreeting = "Good afternoon";

    Greeter greeter = new Greeter(mockDtp.Object);
    string actualGreeting = greeter.Greet();

    Assert.Equal(expectedGreeting, actualGreeting);
}

[Fact]
public void GreeterTest_Evening()
{
    Mock<IDateTimeProvider> mockDtp = new Mock<IDateTimeProvider>();
    mockDtp.SetupGet(it => it.Now).Returns(new DateTime(1, 1, 1, 22, 0, 0)); //10:00 p.m.
    string expectedGreeting = "Good evening";

    Greeter greeter = new Greeter(mockDtp.Object);
    string actualGreeting = greeter.Greet();

    Assert.Equal(expectedGreeting, actualGreeting);
}

Hence the mocking.