How do I determine time passed between two Calendar Objects in Java?

Question

What is the best way to count the amount of time between two Calendar dates in java. I am writing a method that determines the number of months that pass between two dates and returns a boolean based on a predefined term of months. This is my code(does not work correctly).

This code always returns false. Also this code does not take into account the number of days passed. This could be a problem if the start date is at the end of a month. Is there not a simple compareTo method?

private boolean hasMatured()
{
    Calendar now = Calendar.getInstance();

    Calendar start = (Calendar) super.dateOpened.clone();

    int nowYear = now.get(Calendar.YEAR);
    int nowMonth = now.get(Calendar.MONTH);

    int startYear = start.get(Calendar.YEAR);
    int startMonth = start.get(Calendar.MONTH);

    int monthsElapsed = (nowYear - startYear) * 12 + (nowMonth - startMonth);

    return monthsElapsed>PERIOD_IN_MONTHS;
}

Answer 1

int nowYear = now.get(Calendar.YEAR);
int nowMonth = now.get(Calendar.MONTH);

int startYear = now.get(Calendar.YEAR);
int startMonth = now.get(Calendar.MONTH);

int monthsElapsed = (nowYear - startYear) * 12 + (nowMonth - startMonth);

Answer 2

The code looks fine expect from one major caveat: Calendar is mutable.

So, instead of

Calendar start = super.dateOpened;

you should have done

Calendar start = (Calendar) super.dateOpened.clone();

otherwise the changes get reflected in dataOpened which may cause unexpected side-effects.

Answer 3

I would strongly recommend Joda Time for all date-related stuff in Java. It has a much cleaner and more intuitive API, togather with the concepts of intervals between dates etc.

Answer 4

Am I missing something? There is a compareTo() in Calendar, as well as other useful stuff...

How about:

Calendar now = Calendar.getInstance();

now.add(Calendar.MONTH, -PERIOD_IN_MONTHS);

return super.dateOpened.before(now);

Subtract X months from today, and see if the start date is still before that date. If it is, then X months must have passed.