GetTotalMonthDiff

/// <summary>
/// Compute dateTime difference precisely
/// Alex-LEWIS, 2015-08-11
/// </summary>
/// <param name="dt1"></param>
/// <param name="dt2"></param>
/// <returns></returns>
public static double GetTotalMonthDiff(this DateTime dt1, DateTime dt2)
{
  var l = dt1 < dt2 ? dt1 : dt2;
  var r = dt1 >= dt2 ? dt1 : dt2;
  var lDfM = DateTime.DaysInMonth(l.Year, l.Month);
  var rDfM = DateTime.DaysInMonth(r.Year, r.Month);

  var dayFixOne = l.Day == r.Day
    ? 0d
    : l.Day > r.Day
      ? r.Day * 1d / rDfM - l.Day * 1d / lDfM
      : (r.Day - l.Day) * 1d / rDfM;

  return dayFixOne
    + (l.Month == r.Month ? 0 : r.Month - l.Month)
    + (l.Year == r.Year ? 0 : (r.Year - l.Year) * 12);
}

DateTime dt1 = new DateTime(2015, 08, 11);
DateTime dt2 = new DateTime(1992, 10, 10);
var monthTotalDiff = dt1.GetMonthDiff(dt2);