How to calculate actual months difference (calendar year not approximation) between two given dates in C#?

Question

Example: given two dates below, finish is always greater than or equal to start

start = 2001 Jan 01

finish = 2002 Mar 15

So from 2001 Jan 01 to the end of 2002 Feb

months = 12 + 2 = 14

For 2002 March

15/30 = 0.5

so grand total is 14.5 months difference.

It's very easy to work out by hand but how do I code it elegantly? At the moment I have the combination of a lot of if else and while loops to achieve what I want but I believe there are simpler solutions out there.

Update: the output needs to be precise (not approximation) for example: if start 2001 Jan 01 and finish 2001 Apr 16, the output should be 1 + 1 + 1= 3 (for Jan, Feb and Mar) and 16 / 31 = 0.516 month, so the total is 3.516.

Another example would be if I start on 2001 Jul 5 and finish on 2002 Jul 10, the output should be 11 month up to the end of June 2002, and (31-5)/31 = 0.839 and 10/31 = 0.323 months, so the total is 11 + 0.839 + 0.323 = 12.162.

I extended Josh Stodola's code and Hightechrider's code:

public static decimal GetMonthsInRange(this IDateRange thisDateRange)
{
    var start = thisDateRange.Start;
    var finish = thisDateRange.Finish;

    var monthsApart = Math.Abs(12*(start.Year - finish.Year) + start.Month - finish.Month) - 1;

    decimal daysInStartMonth = DateTime.DaysInMonth(start.Year, start.Month);
    decimal daysInFinishMonth = DateTime.DaysInMonth(finish.Year, finish.Month);

    var daysApartInStartMonth = (daysInStartMonth - start.Day + 1)/daysInStartMonth;
    var daysApartInFinishMonth = finish.Day/daysInFinishMonth;

    return monthsApart + daysApartInStartMonth + daysApartInFinishMonth;
}

Answer 1

I gave an int answer before, and then realized what you asked for a more precise answer. I was tired, so I deleted and went to bed. So much for that, I was unable to fall asleep! For some reason, this question really bugged me, and I had to solve it. So here you go...

static void Main(string[] args)
{
    decimal diff;

    diff = monthDifference(new DateTime(2001, 1, 1), new DateTime(2002, 3, 15));
    Console.WriteLine(diff.ToString("n2")); //14.45

    diff = monthDifference(new DateTime(2001, 1, 1), new DateTime(2001, 4, 16));
    Console.WriteLine(diff.ToString("n2")); //3.50

    diff = monthDifference(new DateTime(2001, 7, 5), new DateTime(2002, 7, 10));
    Console.WriteLine(diff.ToString("n2")); //12.16

    Console.Read();
}

static decimal monthDifference(DateTime d1, DateTime d2)
{
    if (d1 > d2)
    {
        DateTime hold = d1;
        d1 = d2;
        d2 = hold;
    }

    int monthsApart = Math.Abs(12 * (d1.Year-d2.Year) + d1.Month - d2.Month) - 1;
    decimal daysInMonth1 = DateTime.DaysInMonth(d1.Year, d1.Month);
    decimal daysInMonth2 = DateTime.DaysInMonth(d2.Year, d2.Month);

    decimal dayPercentage = ((daysInMonth1 - d1.Day) / daysInMonth1)
                          + (d2.Day / daysInMonth2);
    return monthsApart + dayPercentage;
}

Now I shall have sweet dreams. Goodnight :)