class DateTimeDiff
	{
		
		/// <summary>
		///		Difference btweeen dates expressed in years, months and days.
		/// </summary>
		/// <param name="dtStartDate"></param>
		/// <param name="dtEndDate"></param>
		/// <param name="YearsDiff"></param>
		/// <param name="MonthsDiff"></param>
		/// <param name="DaysDiff"></param>
		public static void TimeSpanToDate(
			DateTime dtStartDate, 
			DateTime dtEndDate, 
			out Int32 YearsDiff, 
			out Int32 MonthsDiff, 
			out Int32 DaysDiff
			)
		{
			bool bRegularOrder = true;  // is the start date before end date
			
			if (dtEndDate < dtStartDate) { 
				// if the order of dates is reversed, dates are reversed, then difference is computed
				// however, we remember that order is reversed, so at the end we put minus before all results componenet
				DateTime d3 = dtStartDate;
				dtStartDate = dtEndDate;
				dtEndDate = d3;
				bRegularOrder = false;
			}

			// compute difference in total months
			MonthsDiff = 12 * (dtEndDate.Year - dtStartDate.Year) + (dtEndDate.Month - dtStartDate.Month);

			// based upon the days
			// adjust months & compute actual days difference  
			if (dtEndDate.Day < dtStartDate.Day) {
				// check if both dates are the last day in the month
				if (dtStartDate.Day == DateTime.DaysInMonth(dtStartDate.Year, dtStartDate.Month)
						&& dtEndDate.Day == DateTime.DaysInMonth(dtEndDate.Year, dtEndDate.Month)) {
					DaysDiff = 0; // whereas month number remains unchanged
				}
				else {			
					MonthsDiff--;
					Int32 iBrojDanaPrethodnogMesecaZavrsnogDatuma = DayInMonthOfPreviousMonth(dtEndDate);
					DaysDiff = Math.Max(0, iBrojDanaPrethodnogMesecaZavrsnogDatuma - dtStartDate.Day) 
							+ dtEndDate.Day;
				}
			}
			else {
				DaysDiff = dtEndDate.Day - dtStartDate.Day;
			}
			// compute YearsDiff & actual MonthsDiff  
			YearsDiff = MonthsDiff / 12;
			MonthsDiff -= YearsDiff * 12;

			// if the order was reversed, put minus sign before all componenet results
			if (!bRegularOrder) {
				YearsDiff = -YearsDiff;
				MonthsDiff = -MonthsDiff;
				DaysDiff = -DaysDiff;
			}
		}
		
		private static Int32 DayInMonthOfPreviousMonth(DateTime dt)
		{
			Int32 month = dt.Month;
			Int32 year = dt.Year;
			
			// previous month
			month--;
			if (month == 0) {
				month = 12;
				year--;
			}
			return DateTime.DaysInMonth(year, month);
		}
		
		
	
	}

Datetime difference in years, months and days, corrected version of existing snippet called DateTime Difference provided by hnielsen

DateTime difference 2

C#