Calculating time diffrence

Question

I am trying to calculate time diffrence between dates:

public static void main(String args[])
{
    String start = "2013-03-10 10:28:47.000000";
    String end = "2013-04-07 09:54:08.054577";

    CalculateDuration calc = new CalculateDuration();
    calc.calculateDiffrenceInMinutes(start,end);
}
public int calculateDiffrenceInMinutes(String start_time, String end_time)
{

    SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");  

    Date d1 = null;
    Date d2 = null;
    try {
        d1 = format.parse(start_time);
        d2 = format.parse(end_time);
    } catch (ParseException e) {
        e.printStackTrace();
    }    

    // Get msec from each, and subtract.
    int diff = (int) d2.getTime() - (int)d1.getTime();
    int diffSeconds = (int) (diff / 1000);         
    int diffMinutes = (int) (diff / (60 * 1000));         
    int diffHours = (int) (diff / (60 * 60 * 1000));                      

    return diffMinutes;
}

For some reason i cant understand the example i show give a negetive output even though the end date is after the start date.

Any ideas?

In addition to the answer below, it's generally better to make functions take the actual parameters they're interested in; your method ought to take `Date`s, not `String`s. This makes it easier to test (maybe your conversion wasn't working properly) and more flexible to use. — chrylis -cautiouslyoptimistic-, Sep 02 '13 at 14:13
@chrylis And also, because it might lead to situations later on, where someone converts his `Date` instances to `String` so he can call this method (which then converts them back to `Date`). Which is completeley needless. — brimborium, Sep 02 '13 at 14:21

rocketboy · Accepted Answer · 2013-09-02T14:09:43.403

8

Try changing

int diff = (int) d2.getTime() - (int)d1.getTime();

to

long diff = d2.getTime() - d1.getTime();

Explicit typecasting from long to int will cause precision loss and may result in a negative value on subtraction.

long a = 90000000000000L;
long b = 10000001000000L;

a>b
(int)a-(int)b => negative value

edited Sep 02 '13 at 14:09

answered Sep 02 '13 at 14:02

rocketboy

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Sajal Dutta · Answer 2 · 2013-09-04T08:33:15.320

4

As others pointed out about precision loss, you can read the details in here.

long diff = d2.getTime() - d1.getTime();
long diffSeconds = diff / 1000 % 60;  
long diffMinutes = diff / (60 * 1000) % 60;
long diffHours   = diff / (60 * 60 * 1000) % 60;

Java's date-time API has some design issues. Calendar is not thread safe. You may want to look at Joda-Time. This would make it as simple as-

Seconds.between(startDate, endDate);

edited Sep 04 '13 at 08:33

answered Sep 02 '13 at 14:27

Sajal Dutta

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How is Java's Calendar "very much buggy"? Can you point out some unresolved bugs? That is a bold statement to put in an answer. – VGR Sep 04 '13 at 00:36
@VGR I don't want to write a long message. So here are some answers might help. http://stackoverflow.com/questions/12032051/differences-between-java-util-date-and-joda-time-apis – Sajal Dutta Sep 04 '13 at 08:30
http://stackoverflow.com/questions/12131324/is-java-util-calendar-thread-safe-or-not – Sajal Dutta Sep 04 '13 at 08:31
http://stackoverflow.com/questions/589870/should-i-use-java-date-and-time-classes-or-go-with-a-3rd-party-library-like-joda – Sajal Dutta Sep 04 '13 at 08:32

brimborium · Answer 3 · 2013-09-02T14:15:35.590

Date#getTime() returns a long and that has a reason. Because quite a few milliseconds have passed since January 1, 1970, 00:00:00 GMT. An int can't hold that huge number (and overflows, which explains why you might get negative numbers (depending on the two dates)). So you should use long to do the calculations.

I would change your code like this:

long diff = d2.getTime() - d1.getTime();
long diffSeconds = diff / (          1000);         
long diffMinutes = diff / (     60 * 1000);         
long diffHours   = diff / (60 * 60 * 1000);

Of course, you also need to change the return type of your calculateDiffrenceInMinutes method to long.

Note: In your example, you could also leave diffSeconds, diffMinutes and diffHours as int. But I wouldn't recommend that, as this would be a solution that fails if the dates are too far apart.

Calculating time diffrence

3 Answers3