Unit testing friendly datetime

gistfile1.cs

using System;

namespace Core.Utils
{
    public static class SystemTime
    {
        public static Func<DateTime> Engine { private get; set; }

        static SystemTime()
        {
            SetDefaultEngine();
        }

        public static void SetDefaultEngine()
        {
            Engine = () => DateTime.Now;
        }

        public static DateTime Now
        {
            get
            {
                return Engine();
            }
        }
    }
}