>He says: We still have only one month completed since August 5th and it gives 3
>
>So, how would a timespan fit in - given that the biggest unit a timespan knows about is a Day - no months, no years
>http://msdn.microsoft.com/en-us/library/system.timespan.aspx
>
>Also, a timespan doesn't have a starting point - ie a starting DateTime

OK - we need a DateTimeSpan :-}

   public struct DateTimeSpan
    {
        public DateTimeSpan(int years, int months, int days, int hours, int minutes, int seconds)
        {
            Years = years; Months = months; Days = days;
            Hours = hours; Minutes = minutes; Seconds = seconds;
        }

        public int Years;
        public int Months;
        public int Days;
        public int Hours;
        public int Minutes;
        public int Seconds;

        public override string ToString()
        {
            return string.Format("Years:{0} Months:{1} Days:{2} Hours:{3} Minutes:{4} Seconds:{5}",
                Years, Months, Days, Hours, Minutes, Seconds);
        }
    }

I tried this:

static DateTimeSpan GetDateDiff(DateTime startDate, DateTime endDate)
        {
            int year = endDate.Year;
            int month = endDate.Month;

            TimeSpan startSpan = new TimeSpan(startDate.Day, startDate.Hour, startDate.Minute, startDate.Second);
            TimeSpan endSpan = new TimeSpan(endDate.Day, endDate.Hour, endDate.Minute, endDate.Second);

            if (endSpan - startSpan < new TimeSpan(0))
            {
                month -= 1;
                if (month == 0)
                {
                    month = 12;
                    year -= 1;
                }
            }
            TimeSpan ts = endDate.Subtract(new DateTime(year, month, startDate.Day, startDate.Hour, startDate.Minute, startDate.Second));
            return new DateTimeSpan(year - startDate.Year, month - startDate.Month, ts.Days, ts.Hours, ts.Minutes, ts.Seconds);
        }

Test:

DateTime earlierDate = new DateTime(2006, 1, 12,23, 15, 12);
DateTime laterDate = new DateTime(2010, 1, 4, 9, 55, 40);

            for (int i = 0; i < 1000; i++)
            {
                DateTimeSpan dts = GetDateDiff(earlierDate, laterDate);
                Console.WriteLine("From:{0} To:{1}", earlierDate, laterDate);
                Console.WriteLine(dts);

                if (earlierDate.AddYears(dts.Years).AddMonths(dts.Months).AddDays(dts.Days)
                    .AddHours(dts.Hours).AddMinutes(dts.Minutes).AddSeconds(dts.Seconds) != laterDate)
                    throw new Exception("Crap code");

                laterDate = laterDate.Subtract(new TimeSpan(1, 20, 10));
            }

Maybe could be simplified more but at least GetDateDiff() is a bit shorter...