Joda Time - inconsistent result

Question

I ran a quick test on daysBetween function in Joda-Time library and I am getting inconsistent result. Does anyone else have the same problem?

Below is the code I use for testing.

public static int elapsedDaysJoda() {
    DateTime dt1 = new DateTime(new Date());        
    Calendar cal = Calendar.getInstance();
    cal.set(2011, 0, 1); 
    DateTime dt2 = new DateTime(cal.getTime());
    Days days = Days.daysBetween(dt2, dt1);
    return days.getDays();
}

public static void main(String[] args) {
    for(int i=0; i < 10; i++) {
        System.out.println(elapsedDaysJoda());
    }
}

score 3 · Accepted Answer · edited Jun 10 '19 at 14:00

First, and main problem: you are asking Java for the "current datetime" twice, in new Date() and in Calendar.getInstance(), and you cannot be sure that this two calls will return exactly the same instant or if they can differ in a few milliseconds.

Besides, because you are using DateTime instead of LocalDate, that difference of a few milliseconds can alter the calculation of the elapsed days. If you are (conceptually) dealing with difference between days and you do not care about times (h:m:s.sss) perhaps you should better use LocalDate.

The first problem could be solved by asking for Java current datime only once:

public static int elapsedDaysJoda() {
    Calendar cal = Calendar.getInstance();
    DateTime dt1 = new DateTime(cal);
    cal.set(2011, 0, 1); 
    DateTime dt2 = new DateTime(cal.getTime());
    Days days = Days.daysBetween(dt2, dt1);
    return days.getDays();
}

In addition (or alternatively; this also fixes the inconsistent results), you could use LocalDate instead of DateTime.

Thank you for your clear explanation. LocalDate is what I needed. — carbotex, Jan 04 '12 at 01:43

Joda Time - inconsistent result

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