How to format java.util.Date with DateTimeFormatter portable?

How to format java.util.Date with DateTimeFormatter portable?

I can't use

Date in = readMyDateFrom3rdPartySource();
LocalDateTime ldt = LocalDateTime.ofInstant(in.toInstant(), ZoneId.systemDefault());
ldt.format(dateTimeFormatter);

because I afraid that usage of ZoneId.systemDefault() can introduce some changes.

I need to format exactly that object I have.

UPDATE

Note: time is time. Not space. Timezone is very rough measure of longitude, i.e. space. I don't need it. Only time (and date).

UPDATE 2

I wrote the following program, proving, that Date DOES NOT only contain correct "instant":

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

public class DataNature2 {

   public static void main(String[] args) throws ParseException {

      SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

      String dateTimeString = "1970-01-01 00:00:01";

      Date date = simpleDateFormat.parse(dateTimeString);

      System.out.println("1 second = " + date.getTime());

   }
}

The output is follows:

1 second = -10799000

While it should be

1 second = 1000

if Date was "Instant".

The number 10799000 is 3*60*60*1000-1000 - the timezone offset of my local time.

This means, that Date class is dual. It's millisecond part may be shifted relatively to hh mm ss part by timezone offset.

This means, that if any utility returns Date object in terms of it's parts (hh mm ss) then it implicitly converted to local time. And getTime() means DIFFERENT time simultaneously. I mean on different machines if this program run at the same time, getTime() will be the same, while time parts will be different.

So, the code example in the beginning is correct: it takes "instant" part of Date, and supplies system timezone part, which was implicitly used inside Date. I.e. it converts dual Date object into explicit LocalDateTime object with the same parts. And hence, formatting after that, is correct.

UPDATE 3

Event funnier:

Date date = new Date(70, 0, 1, 0, 0, 1);
assertEquals(1000, date.getTime());

this test fails.

UDPATE 4

New code. Dedicated to all believers.

public class DataNature3 {

   public static class TZ extends java.util.TimeZone {


      private int offsetMillis;

      public TZ(int offsetHours) {
         this.offsetMillis = offsetHours * 60 * 60 * 1000;
      }

      @Override
      public int getOffset(int era, int year, int month, int day, int dayOfWeek, int milliseconds) {
         throw new UnsupportedOperationException();
      }

      @Override
      public void setRawOffset(int offsetMillis) {
         this.offsetMillis = offsetMillis;
      }

      @Override
      public int getRawOffset() {
         return offsetMillis;
      }

      @Override
      public boolean useDaylightTime() {
         return false;
      }

      @Override
      public boolean inDaylightTime(Date date) {
         return false;
      }
   }

   public static void main(String[] args) {

      Date date = new Date(0);

      for(int i=0; i<10; ++i) {

         TimeZone.setDefault(new TZ(i));

         if( i<5 ) {
            System.out.println("I am date, I am an instant, I am immutable, my hours property is " + date.getHours() + ", Amen!");
         }
         else {
            System.out.println("WTF!? My hours property is now " + date.getHours() + " and changing! But I AM AN INSTANT! I AM IMMUTABLE!");
         }

      }

      System.out.println("Oh, please, don't do that, this is deprecated!");

   }
}

Output:

I am date, I am an instant, I am immutable, my hours property is 0, Amen!
I am date, I am an instant, I am immutable, my hours property is 1, Amen!
I am date, I am an instant, I am immutable, my hours property is 2, Amen!
I am date, I am an instant, I am immutable, my hours property is 3, Amen!
I am date, I am an instant, I am immutable, my hours property is 4, Amen!
WTF!? My hours property is now 5 and changing! But I AM AN INSTANT! I AM IMMUTABLE!
WTF!? My hours property is now 6 and changing! But I AM AN INSTANT! I AM IMMUTABLE!
WTF!? My hours property is now 7 and changing! But I AM AN INSTANT! I AM IMMUTABLE!
WTF!? My hours property is now 8 and changing! But I AM AN INSTANT! I AM IMMUTABLE!
WTF!? My hours property is now 9 and changing! But I AM AN INSTANT! I AM IMMUTABLE!
Oh, please, don't do that, this is deprecated!