Question

I am trying to convert from millisecond time stamp to XMLGregorianCalendar and back, but I seem to be getting wrong results. Am I doing something wrong? It seems I am gaining days.

    // Time stamp   01-Jan-0001 00:00:00.000
    Long ts = -62135740800000L;
    System.out.println(ts);
    System.out.println(new Date(ts)); // Sat Jan 01 00:00:00 PST 1 .. Cool!

    // to Gregorian Calendar
    GregorianCalendar gc = new GregorianCalendar();
    gc.setTimeInMillis(ts);

    // to XML Gregorian Calendar
    XMLGregorianCalendar xc = DatatypeFactory.newInstance().newXMLGregorianCalendar(gc);

    // back to GC
    GregorianCalendar gc2 = xc.toGregorianCalendar();

    // to Timestamp
    Long newTs = gc2.getTimeInMillis();
    System.out.println(newTs);    //  -62135568000000  .. uh?
    System.out.println(new Date(newTs));  // Mon Jan 03 00:00:00 PST 1  .. where did the extra days come from?

Answer 1

Interesting – it works fine for values down to (about) -10000000000000L (and positive values) but larger negative values become inconsistent.

If you print out gc, xc, and gc2, you can see where the problem arises (the conversion from XMLGregorianCalendar to GregorianCalendar

gc:  java.util.GregorianCalendar[time=-62135740800000 ... DAY_OF_WEEK=7
xc:  0001-01-01T08:00:00.000Z
gc2: java.util.GregorianCalendar[time=? ... DAY_OF_WEEK=5

If you print out the fields of xc, you get 1,1,1.

    System.out.println(xc.getYear());
    System.out.println(xc.getMonth());
    System.out.println(xc.getDay());

For gc2, you get 1,0,1 (which matches xc, because months are zero-based in GregorianCalendar)

    System.out.println(gc2.get(gc2.YEAR));
    System.out.println(gc2.get(gc2.MONTH));
    System.out.println(gc2.get(gc2.DAY_OF_MONTH));

However, adding these 3 println calls changes the output from printing out gc2! – the time=? output from gc2 changes to time=-62135568000000 – so some calculation has been triggered by querying the GregorianCalendar object; the areFieldsSet property also changes from false to true.

The timezones of the two GregorianCalendars are different, but this does not account for the error, which persists even if you set explicit TimeZone and Locale.