Months from Year Start.

Two possible ways.

((date1.Year - date2.Year) * 12) + date1.Month - date2.Month

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Or, assuming you want an approximate number of 'average months' between the two dates, the following should work for all but very huge date differences.

date1.Subtract(date2).Days / (365.25 / 12)

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With DateTimeSpan, similar to a TimeSpan, except that it includes all the date components in addition to the time components.

Usage:

void Main()
{
    DateTime compareTo = DateTime.Parse("8/13/2010 8:33:21 AM");
    DateTime now = DateTime.Parse("2/9/2012 10:10:11 AM");
    var dateSpan = DateTimeSpan.CompareDates(compareTo, now);
    Console.WriteLine("Years: " + dateSpan.Years);
    Console.WriteLine("Months: " + dateSpan.Months);
    Console.WriteLine("Days: " + dateSpan.Days);
    Console.WriteLine("Hours: " + dateSpan.Hours);
    Console.WriteLine("Minutes: " + dateSpan.Minutes);
    Console.WriteLine("Seconds: " + dateSpan.Seconds);
    Console.WriteLine("Milliseconds: " + dateSpan.Milliseconds);
}

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Timespan stuct as below

public struct DateTimeSpan 
{
    private readonly int years;
    private readonly int months;
    private readonly int days;
    private readonly int hours;
    private readonly int minutes;
    private readonly int seconds;
    private readonly int milliseconds;

    public DateTimeSpan(int years, int months, int days, int hours, int minutes, int seconds, int milliseconds)
    {
        this.years = years;
        this.months = months;
        this.days = days;
        this.hours = hours;
        this.minutes = minutes;
        this.seconds = seconds;
        this.milliseconds = milliseconds;
    }

    public int Years { get { return years; } }
    public int Months { get { return months; } }
    public int Days { get { return days; } }
    public int Hours { get { return hours; } }
    public int Minutes { get { return minutes; } }
    public int Seconds { get { return seconds; } }
    public int Milliseconds { get { return milliseconds; } }

    enum Phase { Years, Months, Days, Done }

    public static DateTimeSpan CompareDates(DateTime date1, DateTime date2) 
    {
        if (date2 < date1)
        {
            var sub = date1;
            date1 = date2;
            date2 = sub;
        }

        DateTime current = date1;
        int years = 0;
        int months = 0;
        int days = 0;

        Phase phase = Phase.Years;
        DateTimeSpan span = new DateTimeSpan();

        while (phase != Phase.Done) 
        {
            switch (phase) 
            {
                case Phase.Years:
                    if (current.AddYears(years + 1) > date2)
                    {
                        phase = Phase.Months;
                        current = current.AddYears(years);
                    }
                    else 
                    {
                        years++;
                    }
                    break;
                case Phase.Months:
                    if (current.AddMonths(months + 1) > date2) 
                    {
                        phase = Phase.Days;
                        current = current.AddMonths(months);
                    }
                    else 
                    {
                        months++;
                    }
                    break;
                case Phase.Days:
                    if (current.AddDays(days + 1) > date2) 
                    {
                        current = current.AddDays(days);
                        var timespan = date2 - current;
                        span = new DateTimeSpan(years, months, days, timespan.Hours, timespan.Minutes, timespan.Seconds, timespan.Milliseconds);
                        phase = Phase.Done;
                    }
                    else 
                    {
                        days++;
                    }
                    break;
            }
        }

        return span;
    }   
}

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I suggest you refer the below link and understand what infrastructure C# provides for date time manipulation.
https://msdn.microsoft.com/en-us/library/bb384267%28v=vs.110%29.aspx

I believe you are looking for the count of "full months", not just the difference between calendar months.

The difference in calendar months is easy to find:

int months = (laterDate.Year - currentDate.Year) * 12 + laterDate.Month - currentDate.Month;

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To find the difference in full months, use a method where you add the months to prevent errors when calculating on the ultimo dates of months, indeed for February:

        /// <summary>
        /// Calculates the difference in full months between the current System.DateTime object and a later date.
        /// </summary>
        /// <param name="currentDate">The date of birth.</param>
        /// <param name="laterDate">The date on which to calculate the age.</param>
        /// <returns>Difference in months to a later day. 0 is returned as minimum.</returns>
        public static int Months(this DateTime currentDate, DateTime laterDate)
        {
            int months;
            months = (laterDate.Year - currentDate.Year) * 12 + laterDate.Month - currentDate.Month;
            if (months > 0)
            {
                if (laterDate.Date < currentDate.Date.AddMonths(months))
                {
                    // The last month is not a full month.
                    // Reduce the month count by one.
                    months--;
                }
            }
            else
            {
                months = 0;
            }
            return months;
        }

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This is an extension method to DateTime. Of course, you can remove static and this to use it as is.

If you wish a count in "decimal months" - which isn't an exact science but will show a difference if either date is moved just one day - you can use this method:

        /// <summary>
        /// Calculates the difference in decimal months between the current System.DateTime object and a later date.
        /// </summary>
        /// <param name="currentDate">The date of birth.</param>
        /// <param name="laterDate">The date on which to calculate the age.</param>
        /// <returns>Difference in decimal months to a later day. 0 is returned as minimum.</returns>
        public static double TotalMonths(DateTime currentDate, DateTime laterDate)
        {
            double months = 0;
            double firstPart = 0;
            double lastPart = 0;

            months = (laterDate.Year - currentDate.Year) * 12 + laterDate.Month - currentDate.Month;
            if (months > 0)
            {
                firstPart = (double)currentDate.Day / DateTime.DaysInMonth(currentDate.Year, currentDate.Month);
                lastPart = (double)laterDate.Day / DateTime.DaysInMonth(laterDate.Year, laterDate.Month);
            }
            else
            {
                months = 0;
            }
            return months - firstPart + lastPart;
        }

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Of course, this can be turned into an extension of DateTime should you prefer so.

/gustav