Clock for Testing/freezing time

gistfile1.cs

public static class Clock
    {
        private static DateTime? _frozenTime;

        /// <summary>
        /// Gets a <see cref="DateTime"/> object that is set to the current date and time on this computer,
        /// expressed as the local time.
        /// </summary>
        public static DateTime Now
        {
            get { return _frozenTime.HasValue ? _frozenTime.Value.ToLocalTime() : DateTime.Now; }
        }

        /// <summary>
        /// Gets the current date.
        /// </summary>
        public static DateTime Today
        {
            get { return Now.Date; }
        }

        /// <summary>
        /// Gets a <see cref="DateTime"/> object that is set to the current date and time on this computer,
        /// expressed as the Coordinated Universal Time (UTC).
        /// </summary>
        public static DateTime UtcNow
        {
            get { return _frozenTime.HasValue ? _frozenTime.Value.ToUniversalTime() : DateTime.UtcNow; }
        }

        /// <summary>
        /// Freezes the clock with the current time.
        /// Until <see cref="Thaw()"/> is called, all calls to <see cref="Now"/>, <see cref="Today"/>, and
        /// <see cref="UtcNow"/> will return the exact same values.
        /// </summary>
        public static DisposableClock Freeze()
        {
            return FreezeLocal(Clock.Now);
        }

        /// <summary>
        /// Freezes the clock with the given date and time, considered to be local time.
        /// Until <see cref="Thaw()"/> is called, all calls to <see cref="Now"/>, <see cref="Today"/>, and
        /// <see cref="UtcNow"/> will return the exact same values.
        /// </summary>
        /// <param name="localDateTime">The local date and time to freeze to</param>
        public static DisposableClock FreezeLocal(DateTime localDateTime)
        {
            if (_frozenTime.HasValue)
                throw new InvalidOperationException("Clock is already frozen");

            _frozenTime = DateTime.SpecifyKind(localDateTime, DateTimeKind.Local);
            return new DisposableClock();
        }

        /// <summary>
        /// Freezes the clock with the given date and time, considered to be Coordinated Universal Time (UTC).
        /// Until <see cref="Thaw()"/> is called, all calls to <see cref="Now"/>, <see cref="Today"/>, and
        /// <see cref="UtcNow"/> will return the exact same values.
        /// </summary>
        /// <param name="utcDateTime">The UTC date and time to freeze to</param>
        public static DisposableClock FreezeUtc(DateTime utcDateTime)
        {
            _frozenTime = DateTime.SpecifyKind(utcDateTime, DateTimeKind.Utc);
            return new DisposableClock();
        }

        /// <summary>
        /// Thaws the clock so that <see cref="Now"/>, <see cref="Today"/>, and <see cref="UtcNow"/>
        /// return normal values.
        /// </summary>
        public static void Thaw()
        {
            if (!_frozenTime.HasValue)
                throw new InvalidOperationException("Clock is not frozen");

            _frozenTime = null;
        }
    }