Right way to convert XMLGregorianCalendar to GregorianCalendar

Question

I have 2 classes. First contains Calendar field and Integer field (tz offset). Second contains XmlGregorianCalendar field. I want to compare date from firs class to date from second.

Calendar cal1 = (Calendar) SerializationUtils.clone(firstClass.getDepartureDatetime());
cal1.add(Calendar.MINUTE, -firstClass.getDepartureTzOffset());

GregorianCalendar cal2 = secondClass.getDepartureDateTime().toGregorianCalendar();
cal2.add(Calendar.MINUTE, -secondClass.getDepartureDateTime().getTimezone());

if (LOGGER.isDebugEnabled()) {
                LOGGER.debug(" - Second  [" + DateFormat.getDateTimeInstance().format(cal2.getTime()) + "]");
                LOGGER.debug(" - First [" + DateFormat.getDateTimeInstance().format(cal1.getTime()) + "]");
}

I've set equal dates (Nov 19, 9:00 AM GMT+1) in that classes.

Depending on the system TZ it shows different results (in GMT TZ):

Debian Lenny, TZ is CET:

Second  [Nov 19, 2011 7:00:00 AM] - wrong!
First [Nov 19, 2011 8:00:00 AM] -right!

Win7, TZ is GMT+3:

Second  [Nov 19, 2011 8:30:00 AM] - wrong!
First [Nov 19, 2011 8:00:00 AM] -right!

What I'm doing wrong?

Thanks.

UPDATE

1st and 2nd classes:

public class FirstClass implements Serializable {
    private static final long serialVersionUID = -1150341618306402800L;

    private Calendar departureDatetime;

    private Integer departureTzOffset;

    public Calendar getDepartureDatetime() {
        return departureDatetime;
    }

    public void setDepartureDatetime(Calendar departureDatetime) {
        this.departureDatetime = departureDatetime;
    }

    public Integer getDepartureTzOffset() {
        return departureTzOffset;
    }

    public void setDepartureTzOffset(Integer departureTzOffset) {
        this.departureTzOffset = departureTzOffset;
    }
}

public class SecondClass implements Serializable
{

    private final static long serialVersionUID = 12345L;

    protected XMLGregorianCalendar departureDateTime;

    public XMLGregorianCalendar getDepartureDateTime() {
        return departureDateTime;
    }

    public void setDepartureDateTime(XMLGregorianCalendar value) {
        this.departureDateTime = value;
    }
}

SerializationUtils is an org.apache.commons.lang.SerializationUtils from Apache commons-lang lib.

Answer 1

It was a problem with Timezone when we call (Calendar) SerializationUtils.clone(firstClass.getDepartureDatetime()). Timezone was set to the server TZ and we lost some hours during comparing.