public  enum DateInterval
{
    Second, Minute, Hour, Day, Week, Month, Quarter, Year
}

public sealed class DateTimeManger
{

    private DateTimeManger()
    {}//end of default constructor

    public static long DateDiff(DateInterval Interval, System.DateTime StartDate,    System.DateTime EndDate )
    {
        long lngDateDiffValue = 0;
        System.TimeSpan TS = new System.TimeSpan(EndDate.Ticks - StartDate.Ticks);
        switch (Interval)
        {
            case DateInterval.Second:
            lngDateDiffValue = (long) TS.TotalSeconds;
            break;
            case DateInterval.Minute:
            lngDateDiffValue = (long) TS.TotalMinutes;
            break;
            case DateInterval.Hour:
            lngDateDiffValue = (long)TS.TotalHours;
            break;                                   
            case DateInterval.Day:
            lngDateDiffValue = (long) TS.Days;
            break;
            case DateInterval.Week:
            lngDateDiffValue = (long)( TS.Days / 7);
            break;
            case DateInterval.Month:
            lngDateDiffValue = (long)( TS.Days / 30);
            break;
            case DateInterval.Quarter:
            lngDateDiffValue = (long)( (TS.Days / 30) / 3 );
            break;
            case DateInterval.Year:
            lngDateDiffValue = (long)( TS.Days / 365);
            break;
        }
        return (lngDateDiffValue);
    }//end of DateDiff

}//end of class

Samples:
public class DateDiffCS
{
    public static void Main (string[] CmdLineArgs)
    {
        System.DateTime date1 = System.DateTime.Now;
        System.DateTime date2 = System.DateTime.Now.AddYears(2);

        System.Console.Out.WriteLine("Days : " + DateTimeManger.DateDiff
        (DateTimeManger.DateInterval.