C# - Calculate difference between two dates and get value in month(s)

I use c#, V2005
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How I can get difference between two dates and get value in month(s)
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I had found some solutions but it is not exactly what I need.
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private static int monthDifference(DateTime startDate, DateTime
endDate)
{
int monthsApart = 12 * (startDate.Year - endDate.Year) +
startDate.Month - endDate.Month;
return Math.Abs(monthsApart);
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}
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on this way I can get int values.
For me is important to get value for full and half month, for e.g. 1
month and 15 days...
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1 month, 1.5 month, 2, 2.5, 3, 3.5 etc. Actually, I want to get some
values from 15 days to 12 months and cover all possibilities 15 days
+15 days +15 days+
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something like this: 15 days, 30, 45, 60, ...etc.
Is that possible

[...]
1 month, 1.5 month, 2, 2.5, 3, 3.5 etc. Actually, I want to get some
values from 15 days to 12 months and cover all possibilities 15 days
+15 days +15 days+
>
something like this: 15 days, 30, 45, 60, …etc.
Is that possible

[...]
It is related on the duration of marketing campaign. That duration can
be half month, one, one and half, two months etc. User make choice
about it based on months (0.5; 1; 1.5;2;2.5 month etc.) and
"FromDate".
User makes choice how many months and days he wants to take for
campaign. He can choose from 1 to 24 months and 0 or 15 days. I have
two DateTimePicker controls and after he makes choice about duration
and "FromDate" I use Value.AddMonths and AddDays to calculate
"ToDate" and write the both values in DB. [...]
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May be this is not a good way to achieve this problem, In that case
please suggest better apporach.

>On Sat, 15 Dec 2007 00:42:02 -0800, Mike <ablyplus@yahoo.comwrote:
>>
>Well, maybe I'm missing something but what I would do is store the
>original user selection (the start date, along with the duration in months
>and a 0 or 15 day fraction), rather than computing the "ToDate" and
>storing that. It's trivial to compute the "ToDate" any time you need it
from the original user input. But it is more complicated to go the other
>direction. Not impossible, but certainly harder and given that the
>calculations are really to be based on the user's original input, I think
>it makes more sense to just store that in the database instead.
>
>All that said, as far as your original question goes, you could use the
>code you posted originally, and then add some statements to deal with the
>possible half-month. Presumably if the user selected a whole-month
>duration, the day-of-month (DateTime.Day) for the "FromDate" and "ToDate"
>will be the same. So if they are different, you know you're dealing with
>a half-month situation. In that case, it's simply a matter of comparing
>the day-of-month for the start and end date; if the start day-of-month is
>less than the end day-of-month, you need to add 0.5 to the total month
>calculation, and if it's greater, you need to subtract 0.5.
>
>But like I said, that's a lot more complicated code than just adding a
>number of months and 0 or 15 days. Your code will be much more
>maintainable IMHO if you do it the simple way.
>
>Pete