range between dates...

    public static void DateDiff(DateTime dt1, DateTime dt2, out int years, out int months, out int days)

    {

       int dv1 = dt1.Year * 372 + (dt1.Month - 1) * 31 + dt1.Day - 1;

       int dv2 = dt2.Year * 372 + (dt2.Month - 1) * 31 + dt2.Day - 1;

       days = Math.Abs(dv2 -dv1);

       months = days / 31;

       years = months / 12;

       months = months % 12;

       if (dv2 >= dv1)

       {

          if (dt2.Day >= dt1.Day)

          {

              days = dt2.Day - dt1.Day;

          }

          else

          {

              int dim = DateTime.DaysInMonth(dt1.Year, dt1.Month);

              days = dt2.Day + dim - dt1.Day;

          } 

       }

       else

       {

          if (dt1.Day >= dt2.Day)

          {

              days = dt1.Day - dt2.Day;

          }

          else

          {

              int dim = DateTime.DaysInMonth(dt2.Year, dt2.Month);

              days = dt1.Day + dim - dt2.Day;

          } 

       }

}     

 DateTime dt1 = new DateTime(2000, 2, 10);  // 10 February 2000

 DateTime dt2 = DateTime.Today; // 2 April 2014

 int years, months, days;

 DateDiff(dt1, dt2, out years, out months, out days);
 string result = String.Format("{0} years, {1} months, {2} days", years, months, days);
 Response.Write(result);

The output is:

14 years, 1 months, 21 days

The method always returns the positive difference between two dates and so it doesn't matter in what order they are presented to it.

In your case the parameters will, of course, be:

DateTime dt1 = DateTimePicker1.Value.Date;
DateTime dt2 = DateTimePicker2.Value.Date;