This has been discussed to death, but I still can't get my head around it.
I want to compute a day number from a date and then get back the date.
Could anyone please point out where I'm going wrong? I guess I'm
misunderstanding something about Calendar offsets, or about the
GregorianCalendar constructors.

-- Thanks, Sebastian

Here's how I compute the number of days that have passed in local time
since Jan 1, 1970. (seems to work fine):

public class MyCalendar extends GregorianCalendar
{
 private static final long MILLISECONDS_PER_DAY = 86400000L;
 
 public MyCalendar()  {
   super();
 }
 
 public MyCalendar(int year, int month, int day) {
   super(year, month, day);
 }

 public long getDayNumber() {
   // adjust from utc to local time adding the Calendar offset values
   long offset = get( Calendar.ZONE_OFFSET ) + get( Calendar.DST_OFFSET );
   long day = ( long )Math.floor( ( double )( getTimeInMillis() + offset )
                       / ( ( double )MILLISECONDS_PER_DAY ) );
   return day;
 }
}

However, the inverse method (creating a Calendar from a day number)
doesn't work as I expect:

 public static MyCalendar createInstanceDayNumber(long dn)
 {
   MyCalendar cal = new MyCalendar();
   long offset = cal.get( Calendar.ZONE_OFFSET )
                 + cal.get( Calendar.DST_OFFSET );
   // adjust from local time to utc subtracting the Calendar offset values
   long utcTime = ( dn * MILLISECONDS_PER_DAY ) - offset;
   cal.setTimeInMillis(utcTime);
 
   return cal;
 }

With this Unit Test:

 @Test
 public void testCreateInstanceDayNumber()
 {
   SimpleDateFormat df = new SimpleDateFormat("dd.MM.yyyy");
   MyCalendar epoch = new MyCalendar(1970, Calendar.JANUARY, 1);
   MyCalendar cal = MyCalendar.createInstanceDayNumber(0);
   assertEquals(df.format(epoch.getTime()), df.format(cal.getTime()));
 }

org.junit.ComparisonFailure: null expected:<[01.01.1970]> but
was:<[31.12.1969]>