calculating the difference in months between two dates

Question

You won't be able to get that from a TimeSpan, because a "month" is a variable unit of measure. You'll have to calculate it yourself, and you'll have to figure out how exactly you want it to work.

For example, should dates like July 5, 2009 and August 4, 2009 yield one month or zero months difference? If you say it should yield one, then what about July 31, 2009 and August 1, 2009? Is that a month? Is it simply the difference of the Month values for the dates, or is it more related to an actual span of time? The logic for determining all of these rules is non-trivial, so you'll have to determine your own and implement the appropriate algorithm.

If all you want is simply a difference in the months--completely disregarding the date values--then you can use this:

public static int MonthDifference(this DateTime lValue, DateTime rValue)
{
    return (lValue.Month - rValue.Month) + 12 * (lValue.Year - rValue.Year);
}

Note that this returns a relative difference, meaning that if rValue is greater than lValue, then the return value will be negative. If you want an absolute difference, you can use this:

public static int MonthDifference(this DateTime lValue, DateTime rValue)
{
    return Math.Abs((lValue.Month - rValue.Month) + 12 * (lValue.Year - rValue.Year));
}

(I realize this is an old question, but...)

This is relatively painful to do in pure .NET. I'd recommend my own Noda Time library, which is particularly designed for things like this:

LocalDate start = new LocalDate(2009, 10, 6);
LocalDate end = new LocalDate(2009, 12, 25);
Period period = Period.Between(start, end);
int months = period.Months;

(There are other options, e.g. if you only want a count of months even across years, you'd use Period period = Period.Between(start, end, PeriodUnits.Months);)

Maybe you don't want to know about month fractions; What about this code?

public static class DateTimeExtensions
{
    public static int TotalMonths(this DateTime start, DateTime end)
    {
        return (start.Year * 12 + start.Month) - (end.Year * 12 + end.Month);
    }
}

//  Console.WriteLine(
//     DateTime.Now.TotalMonths(
//         DateTime.Now.AddMonths(-1))); // prints "1"

I've written a very simple extension method on DateTime and DateTimeOffset to do this. I wanted it to work exactly like a TotalMonths property on TimeSpan would work: i.e. return the count of complete months between two dates, ignoring any partial months. Because it's based on DateTime.AddMonths() it respects different month lengths and returns what a human would understand as a period of months.

(Unfortunately you can't implement it as an extension method on TimeSpan because that doesn't retain knowledge of the actual dates used, and for months they're important.)

The code and tests are both available on GitHub. The code is very simple:

public static int GetTotalMonthsFrom(this DateTime dt1, DateTime dt2)
{
    DateTime earlyDate = (dt1 > dt2) ? dt2.Date : dt1.Date;
    DateTime lateDate = (dt1 > dt2) ? dt1.Date : dt2.Date;

    // Start with 1 month's difference and keep incrementing
    // until we overshoot the late date
    int monthsDiff = 1;
    while (earlyDate.AddMonths(monthsDiff) <= lateDate)
    {
        monthsDiff++;
    }

    return monthsDiff - 1;
}

And it passes all these unit test cases:

// Simple comparison
Assert.AreEqual(1, new DateTime(2014, 1, 1).GetTotalMonthsFrom(new DateTime(2014, 2, 1)));
// Just under 1 month's diff
Assert.AreEqual(0, new DateTime(2014, 1, 1).GetTotalMonthsFrom(new DateTime(2014, 1, 31)));
// Just over 1 month's diff
Assert.AreEqual(1, new DateTime(2014, 1, 1).GetTotalMonthsFrom(new DateTime(2014, 2, 2)));
// 31 Jan to 28 Feb
Assert.AreEqual(1, new DateTime(2014, 1, 31).GetTotalMonthsFrom(new DateTime(2014, 2, 28)));
// Leap year 29 Feb to 29 Mar
Assert.AreEqual(1, new DateTime(2012, 2, 29).GetTotalMonthsFrom(new DateTime(2012, 3, 29)));
// Whole year minus a day
Assert.AreEqual(11, new DateTime(2012, 1, 1).GetTotalMonthsFrom(new DateTime(2012, 12, 31)));
// Whole year
Assert.AreEqual(12, new DateTime(2012, 1, 1).GetTotalMonthsFrom(new DateTime(2013, 1, 1)));
// 29 Feb (leap) to 28 Feb (non-leap)
Assert.AreEqual(12, new DateTime(2012, 2, 29).GetTotalMonthsFrom(new DateTime(2013, 2, 28)));
// 100 years
Assert.AreEqual(1200, new DateTime(2000, 1, 1).GetTotalMonthsFrom(new DateTime(2100, 1, 1)));
// Same date
Assert.AreEqual(0, new DateTime(2014, 8, 5).GetTotalMonthsFrom(new DateTime(2014, 8, 5)));
// Past date
Assert.AreEqual(6, new DateTime(2012, 1, 1).GetTotalMonthsFrom(new DateTime(2011, 6, 10)));

You will have to define what you mean by TotalMonths to start with.
A simple definition puts a month at 30.4 days (365.25 / 12).

Beyond that, any definition including fractions seems useless, and the more common integer value (whole months between dates) also depends on non-standard business rules.

You need to work it out yourself off the datetimes. How you deal with the stub days at the end will depend on what you want to use it for.

One method would be to count month and then correct for days at the end. Something like:

   DateTime start = new DateTime(2003, 12, 25);
   DateTime end = new DateTime(2009, 10, 6);
   int compMonth = (end.Month + end.Year * 12) - (start.Month + start.Year * 12);
   double daysInEndMonth = (end - end.AddMonths(1)).Days;
   double months = compMonth + (start.Day - end.Day) / daysInEndMonth;