Why does GregorianCalendar.getTimeInMillis() return wrong value?

I want to calculate time gap in form of days between two dates. So I wrote following code:

public static int calculateTimeGap(long start, long end) {
    GregorianCalendar calendar1 = new GregorianCalendar();
    calendar1.setTimeInMillis(start);
    System.out.println("calendar1:");
    showCalendar(calendar1);

    GregorianCalendar calendar2 = new GregorianCalendar();
    calendar2.setTimeInMillis(end);
    System.out.println("calendar2:");
    showCalendar(calendar2);

    int gap = (int) (end / 86400000 - start / 86400000);
    System.out.println("gap in days:" + gap);
    return gap;
}

Here are outputs:

03-02 15:28:08.021 20814-20814/com.ywwynm.everythingdone I/System.out: calendar1:
03-02 15:28:08.021 20814-20814/com.ywwynm.everythingdone I/System.out: year: 2016
03-02 15:28:08.021 20814-20814/com.ywwynm.everythingdone I/System.out: month: 3
03-02 15:28:08.021 20814-20814/com.ywwynm.everythingdone I/System.out: day: 2
03-02 15:28:08.021 20814-20814/com.ywwynm.everythingdone I/System.out: week: 3
03-02 15:28:08.021 20814-20814/com.ywwynm.everythingdone I/System.out: hour: 15
03-02 15:28:08.021 20814-20814/com.ywwynm.everythingdone I/System.out: minute: 28
03-02 15:28:08.021 20814-20814/com.ywwynm.everythingdone I/System.out: second: 8
03-02 15:28:08.021 20814-20814/com.ywwynm.everythingdone I/System.out:  
03-02 15:28:08.021 20814-20814/com.ywwynm.everythingdone I/System.out: calendar2:
03-02 15:28:08.021 20814-20814/com.ywwynm.everythingdone I/System.out: year: 2016
03-02 15:28:08.021 20814-20814/com.ywwynm.everythingdone I/System.out: month: 3
03-02 15:28:08.022 20814-20814/com.ywwynm.everythingdone I/System.out: day: 3
03-02 15:28:08.022 20814-20814/com.ywwynm.everythingdone I/System.out: week: 4
03-02 15:28:08.022 20814-20814/com.ywwynm.everythingdone I/System.out: hour: 1
03-02 15:28:08.022 20814-20814/com.ywwynm.everythingdone I/System.out: minute: 0
03-02 15:28:08.022 20814-20814/com.ywwynm.everythingdone I/System.out: second: 0
03-02 15:28:08.022 20814-20814/com.ywwynm.everythingdone I/System.out:  
03-02 15:28:08.022 20814-20814/com.ywwynm.everythingdone I/System.out: gap in days:0

You can see that two GregorianCalendars present different days, but finally we got a gap equals 0.

The parameters of this method, start and end, come from following code:

GregorianCalendar curCalendar = new GregorianCalendar();
long start = curCalendar.getTimeInMillis(); // parameter start

GregorianCalendar calendar = new GregorianCalendar(2016, Calendar.MARCH, 3, 1, 0);
long end = calendar.getTimeInMillis(); // parameter end

So the behavior is very strange. I thought that gregorianCalendar.getTimeInMillis() will always return correct and timezone-independent value so that it can be used as a stable and trustworthy standard to compute and compare time. However, as the code and output shown, it is wrong. And if I considered my timezone(Beijing GMT+8:00), add an offset by Timezone.getDefault().getRawOffset() to parameter start or minus it to end, the result will be correct. But this is actually strange.

What made me get incorrect result?

UPDATE:

public static void showCalendar(GregorianCalendar calendar) {
    System.out.println("year: "   +  calendar.get(Calendar.YEAR));
    System.out.println("month: "  + (calendar.get(Calendar.MONTH) + 1));
    System.out.println("day: "    +  calendar.get(Calendar.DATE));
    System.out.println("week: "   + (calendar.get(Calendar.DAY_OF_WEEK) - 1));
    System.out.println("hour: "   +  calendar.get(Calendar.HOUR_OF_DAY));
    System.out.println("minute: " +  calendar.get(Calendar.MINUTE));
    System.out.println("second: " +  calendar.get(Calendar.SECOND));
    System.out.println(" ");
}