Month diff calculator

DateTimeExtensionMethods.cs

public static class DateTimeExtensionMethods 
{
  public static int GetMonthsDiff(this DateTime fromDate, DateTime toDate)
  {
    if (fromDate > toDate)
    {
        var aux = fromDate;
        fromDate = toDate;
        toDate = aux;
    }

    var monthDiff = (toDate.Year - fromDate.Year) * 12 + (toDate.Month - fromDate.Month);
    var fromTimeIsLaterThanToTime = fromDate.AddMonths(monthDiff) > toDate;
    if (fromTimeIsLaterThanToTime || !IsFullMonthDiff(fromDate, toDate))
    {
        monthDiff--;
    }

    return monthDiff;
  }
  
  public static bool IsLastDayOfMonth(this DateTime date)
  {
    return DateTime.DaysInMonth(date.Year, date.Month) == date.Day;
  }
  
  private static bool IsFullMonthDiff(DateTime fromDate, DateTime toDate)
  {
    return toDate.Day >= fromDate.Day || (toDate.IsLastDayOfMonth() && fromDate.IsLastDayOfMonth());
  }
}