ApplicationTime.cs

public static class ApplicationTime
{
    private static readonly Func<DateTime> _defaultLogic = () => DateTime.UtcNow;

    private static Func<DateTime> _current = _defaultLogic;

    /// <summary>
    /// Returns current date/time, correct in application context
    /// </summary>
    /// <remarks>Normally this would return <see cref="DateTime.UtcNow"/>, but can be changed to make testing easier.</remarks>
    public static DateTime Current
    {
        get { return _current(); }
    }

    /// <summary>
    /// Changes logic behind <see cref="Current"/>. Useful in scenarios where time needs to be defined upfront, like unit tests.
    /// </summary>
    /// <remarks>Be sure you know what you are doing when calling this method.</remarks>
    [EditorBrowsable(EditorBrowsableState.Advanced)]
    public static void _replaceCurrentTimeLogic(Func<DateTime> newCurrentTimeLogic)
    {
        _current = newCurrentTimeLogic;
    }

    /// <summary>
    /// Reverts current logic to the default logic. Useful in scenarios where unit test changed logic and should rever system to previous state.
    /// </summary>
    /// <remarks>Be sure you know what you are doing when calling this method.</remarks>
    [EditorBrowsable(EditorBrowsableState.Advanced)]
    public static void _revertToDefaultLogic()
    {
        _current = _defaultLogic;
    }
}