Public static funcs wrapping system calls.

SystemDate.cs

namespace System
{
    public static class SystemDate
    {
        public static Func<DateTime> UtcNow { get; private set; }

        static SystemDate()
        {
            Reset();
        }

        private static void Reset()
        {
            UtcNow = () => DateTime.UtcNow;
        }

        public static IDisposable SetUtcNow(Func<DateTime> replacement)
        {
            UtcNow = replacement;
            return new DisposableAction(Reset);
        }
    }
}

SystemThread.cs

namespace System.Threading
{
    public static class SystemThread
    {
        public static Action<TimeSpan> Sleep { get; private set; }

        static SystemThread()
        {
            Reset();
        }

        private static void Reset()
        {
            Sleep = timespan => Thread.Sleep(timespan);
        }

        public static IDisposable SetSleep(Action<TimeSpan> replacement)
        {
            Sleep = replacement;
            return new DisposableAction(Reset);
        }
    }
}

TestSystemThread.cs

using System;
using System.Threading;
using Machine.Specifications;

namespace Omnyx.Scanner.Specifications.Infrastructure
{
    public class TestSystemThread : IAssemblyContext
    {
        public static TimeSpan Slept { get; private set; }
        
        private IDisposable _reset;

        public void OnAssemblyStart()
        {
            // Disable sleeping and stash off the timespan for easy assertions.
            _reset = SystemThread.SetSleep(timespan => { Slept = timespan; });
        }

        public void OnAssemblyComplete()
        {
            _reset.Dispose();
            Slept = TimeSpan.MinValue;
        }
    }
}