SystemDateTime - a mockable implementation of System.DateTime.Now

SystemDateTime.cs

/// <summary>
    /// Provides a mockable implementation of System.DateTime.Now;
    /// </summary>
    public static class SystemDateTime
    {
        private static readonly Func<DateTime> DefaultProvider = () => DateTime.Now;

        [ThreadStatic]
        private static Func<DateTime> dateTimeProvider;

        static SystemDateTime()
        {
            dateTimeProvider = DefaultProvider;
        }

        /// <summary>
        /// Gets the current DateTime
        /// </summary>
        public static DateTime Now
        {
            get
            {
                return dateTimeProvider();
            }
        }

        /// <summary>
        /// Allows mocking of system date time in a using() block
        /// </summary>
        public static IDisposable MockDateTime(DateTime value)
        {
            dateTimeProvider = () => value;
            return new ActionDisposable(() => dateTimeProvider = DefaultProvider);
        }

        /// <summary>
        /// Creates an IDisposable that calls the given function when disposed.
        /// Used to reset state to an initial value once a using() block completes.
        /// </summary>
        private class ActionDisposable : IDisposable
        {
            private readonly Action disposedCallback;

            public ActionDisposable(Action disposedCallback)
            {
                this.disposedCallback = disposedCallback;
            }

            /// <summary>
            /// Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.
            /// </summary>
            public void Dispose()
            {
                disposedCallback();
            }
        }
    }

Tests.cs

[Fact]
public void can_mock_dateTime()
{
    var mockDate = new DateTime(2014, 01, 14);

    using (SystemDateTime.MockDateTime(mockDate))
    {
        Assert.Equal(mockDate, SystemDateTime.Now);
    }

    Assert.NotEqual(mockDate, SystemDateTime.Now);
}