Java Calendar: Calculate Number of Months

Hello Experts:

I'm trying to accurately calculate the number of months between two dates and I"m running into a tricky situation:

Let's say I have two dates:
start: 11/01/2010
end: 12/02/2010

I converted these dates strings into a Date object.  Now, I'm using the GregorianCalendar to calculate the number of months like so:

int months = ((end.get(Calendar.YEAR) - start.get(Calendar.YEAR)) *  end.getMaximum(Calendar.MONTH) ) + end.get(Calendar.MONTH) - start.get(Calendar.MONTH);

This gives me the correct number of months, which is 1.

But if I change the dates like so:
start: 11/15/2010
end: 12/01/2010

I still get the number of months as 1, when really it should be 0.

Can anyone suggest a different and accurate way to calculate the number of months between two dates?  

Thank you!
Greengiants15Asked:
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HackneyCabConnect With a Mentor Commented:
I was bored, so I put this together. It satisfies all of your example dates and results, but you probably want to improve the error checking (such as better checking that the end date is actually later than the start date). Oh, and you'll need to change the exception handling, because my instant response to exceptions is to terminate the JVM.

public static int wholeMonthsPassed(String startDate, String endDate) {
	DateFormat df = new SimpleDateFormat("y/M/d");
	GregorianCalendar start = new GregorianCalendar();
	GregorianCalendar end = new GregorianCalendar();
	try {
		start.setTime(df.parse(startDate));
		end.setTime(df.parse(endDate));
	} catch (ParseException pe) {
		System.out.println("Failed to parse one or both date strings!");
		System.exit(1);
	}
	int startYear, startMonth, startDay;
	int endYear, endMonth, endDay;
	startYear = start.get(Calendar.YEAR);
	startMonth = start.get(Calendar.MONTH);
	startDay = start.get(Calendar.DAY_OF_MONTH);

	endYear = end.get(Calendar.YEAR);
	endMonth = end.get(Calendar.MONTH);
	endDay = end.get(Calendar.DAY_OF_MONTH);

	int yearsInMonths = 12 * (endYear - startYear);

	endMonth += yearsInMonths;

	int monthsPassed = endMonth - startMonth;

	if(endDay < startDay) {
		--monthsPassed;
	}

	if(monthsPassed < 0) {
		System.out.println("Cannot have end year earlier than start year!");
		System.exit(1);
	}

	return monthsPassed;
}

public static void main(String[] args) {
	String start = "2010/10/25";
	String end =   "2011/12/15";

	System.out.println("Result: "+wholeMonthsPassed(start, end));
}

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CEHJCommented:
Define 'month' please so that we help accurately
0
 
ksivananthCommented:
try this,

int noOfMonth = TimeUnit.MONTH.convert( date1.getTime() - date2.getTime(), TimeUnit.MILLISECONDS ) ;
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Greengiants15Author Commented:
Month would be the total number of months.

For example:
10/15/2010
to
12/15/2011

The number of months 14.

but
10/25/2010
to
12/15/2011
would be 13 months
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CEHJCommented:
>>TimeUnit.MONTH

Which version of Java has the above?
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CEHJCommented:
>>Month would be the total number of months.

That's circular, so can't be used as the definition of month
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HainKurtConnect With a Mentor Sr. System AnalystCommented:
add a condition after you get the month:

if (day2<day1) {month = month - 1}
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Greengiants15Author Commented:
I'm running java 1.4.  TimeUnit is only good with 1.5
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CEHJCommented:
>>TimeUnit is only good with 1.5

That's academic. afaics TimeUnit.MONTH doesn't exist
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Greengiants15Author Commented:
HainKurt: I like your suggestion.  That works but then it brings up another problem.

If the date is the following:
start: 10/5/2010
end: 11/1/2010

The number of months comes out as 0 (using your calculations) but if I try to calculate the number of days (start.get(Calendar.DAY_OF_MONTH) - end.get(Calendar.DAY_OF_MONTH))
I get 4 when I should get 27.
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HackneyCabCommented:
Oh, I should point out that my code is currently configured to parse dates in the most sensible format: year/month/date so if you must use the (very confusing) US format month/date/year, you'll need to change the call to SimpleDateFormat from ("y/M/d") to ("M/d/y").
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HainKurtSr. System AnalystCommented:
you get 4 (5-1 = 4)

if (start.get(Calendar.DAY_OF_MONTH) > end.get(Calendar.DAY_OF_MONTH)) {month = month - 1};

it does not calculate how many days between two dates, just checks the day number...
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