using System;

class SystemTime
{
    private static Func<DateTime> _defaultTimeSource = () => DateTime.UtcNow;
    private static Func<DateTime> _timeSource = _defaultTimeSource;
    public static DateTime UtcNow => _timeSource();

    public static IDisposable Freeze(DateTime? time = null)
    {
        var frozen = time ?? DateTime.UtcNow;
        _timeSource = () => frozen;
        return Resetter.Instace;
    }

    private static void Reset() => _timeSource = _defaultTimeSource;

    private class Resetter : IDisposable
    {
        public static readonly Resetter Instace = new Resetter();

        public void Dispose()
        {
            SystemTime.Reset();
        }
    }
}

class Program
{
    static void Main(string[] args)
    {
        using (SystemTime.Freeze(new DateTime(2015, 10, 2)))
        {
            Console.WriteLine(SystemTime.UtcNow);
        }

        Console.WriteLine(SystemTime.UtcNow);
    }
}