Calculate months between two dates.

MonthsBetweenTwoDates.cs

private int GetMonthsBetweenTwoDates(DateTime startDate, DateTime endDate)
{
    int monthsCountResult = 0;
    var monthsCount = 12;
    if (startDate.Year == endDate.Year)
    {
        monthsCountResult = (endDate.Month - startDate.Month) + 1;
    }
    else
    {
        int monthsStartDate = (monthsCount - startDate.Month) + 1;
        int monthsEndDate = endDate.Month;
        int middleYearsMonths = ((endDate.Year - startDate.Year) - 1) * monthsCount;

        monthsCountResult = monthsStartDate + monthsEndDate + middleYearsMonths;
    }

    return monthsCountResult;
}

// Or
/*private int GetMonthsBetweenTwoDates(DateTime startDate, DateTime endDate)
{
    endDate.Substract(startDate).Days / 30;
}*/

protected virtual int GetMonthsBetweenTwoDates(DateTime startDate, DateTime endDate, bool inclusive = true)
{
    if (endDate.Year == startDate.Year && endDate.Month == startDate.Month)
    {
        // Because the logic for Jan 2016 - Jan 2016 returns 2
        return 1;
    }

    // int theMonths = (endDate.Subtract(startDate).Days / 30) + 1;
    int theMonths = ((endDate.Year - startDate.Year) * 12) + endDate.Month - startDate.Month;

    if (inclusive)
    {
        theMonths++;
    }

    return theMonths;
}