Answer by Saeed Mahmoudi for Difference in months between two dates

The most precise way is this that return difference in months by fraction :

private double ReturnDiffereceBetweenTwoDatesInMonths(DateTime startDateTime, DateTime endDateTime){    double result = 0;    double days = 0;    DateTime currentDateTime = startDateTime;    while (endDateTime > currentDateTime.AddMonths(1))    {        result ++;        currentDateTime = currentDateTime.AddMonths(1);    }    if (endDateTime > currentDateTime)    {        days = endDateTime.Subtract(currentDateTime).TotalDays;    }    return result + days/endDateTime.GetMonthDays;}