LogEntryRefactored.cs

public class LogEntry
{
    class DefaultCurrentDateProvider
        : ICurrentDateProvider
    {
        public DateTime GetCurrentDate()
        {
            return DateTime.Now;
        }
    }

    public LogEntry(
        string message, 
        IOutput output = null, 
        ICurrentDateProvider currentDateProvider = null ) 
    {
        this.Message = message;
        if (currentDateProvider == null)
            currentDateProvider = new DefaultCurrentDateProvider();

        this.When = currentDateProvider.GetCurrentDate();
        if (output != null)
            output.Write(this);
    }

    public string Message { get; private set; }
    public DateTime When { get; private set; }
}