Finding Date Difference

What's th开发者_运维知识库e efficient way of finding say if a date is 5 days earlier than another day? Do I need to parse both days using a particular SimpleDateFormat first before I compare?

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The best Java date time API is Joda Time. It makes these tasks, and others, much easier than using the standard API.

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Most quickly, but least accurately, you might just put both into a java.util.Date, getTime() on both, and divide the difference by the number of milliseconds in a day.

You could make it a bit more accurate by creating two Calendar objects, and work with those.

If you really want to solve this well, and have a good bit of time on your hands, look at Joda Time.

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The Calendar interface has some nice methods, including before, after, and equals.

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http://www.java2s.com/Code/Java/Development-Class/DateDiffcomputethedifferencebetweentwodates.htm

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You can do

Long DAY_IN_MILLIS = 86400000;

if((dateA - dateB) > DAY_IN_MILLIS * 5) {
    // dateA is more than 5 days older than dateB
}

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Date date1 = // whatever
Date date2 = // whatever

Long fiveDaysInMilliseconds = 1000 * 60 * 60 * 24 * 5    
boolean moreThan5Days = Math.abs(date1.getTime() - date2.getTime()) > fiveDaysInMilliseconds 

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If you need to ignore the time of the dates, you can do something like

    public static boolean compare5days(Date date, Date another) {
        Calendar cal = Calendar.getInstance();
        cal.setTime(another);
        cal.add(Calendar.DATE, -5);

        // clear time 
        cal.set(Calendar.HOUR_OF_DAY, 0);
        cal.set(Calendar.MINUTE, 0);
        cal.set(Calendar.SECOND, 0);
        cal.set(Calendar.MILLISECOND, 0);

        return date.before(cal.getTime());
    }

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days = (date2.getTime() - date1.getTime())/86400000L

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days = (date2.getTime() - date1.getTime())