Java time: starts at 1th January 1970 at 1am?

Question

What am I missing? Date() is the number of milliseconds that have elapsed since midnight, January 1, 1970. Should midnight not start at 0am?

References:

• http://www.tutorialspoint.com/java/java_date_time.htm
• https://docs.oracle.com/javase/8/docs/api/java/time/package-summary.html
• https://docs.oracle.com/javase/7/docs/api/java/sql/Time.html

My test program:

package be.test.package.time;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.Locale;

public class TimeWork {

    public static void main(String[] args) {    

        List<Long> longs = new ArrayList<>();
        List<String> strings = new ArrayList<>();

        DateFormat formatter = new SimpleDateFormat("dd-MM-yyyy hh:mm:ss.SSS");

        //Now
        Date now = new Date();
        strings.add(formatter.format(now));

        //Test dates
        strings.add("01-01-1970 00:00:00.000");
        strings.add("01-01-1970 01:00:00.000");
        strings.add("31-11-1969 00:00:00.000");
        strings.add("01-01-2014 00:00:00.000");

        //Test data
        longs.add(-1L);
        longs.add(0L);
        longs.add(1L);
        longs.add(7260000L);
        longs.add(1417706084037L);
        longs.add(-7260000L);

        //Show the long value of the date
        for (String string: strings) {
            try {
                Date date = formatter.parse(string);
                System.out.println("Formated date : " + string + " = Long = " + date.getTime());
            } catch (ParseException e) {
                e.printStackTrace();
            }
        }

        //Show the date behind the long
        for (Long lo : longs) {
            Date date = new Date(lo);
            String string = formatter.format(date);
            System.out.println("Formated date : " + string + " = Long = " + lo);        
        }
    }
}

This are the results:

Formated date : 05-12-2014 08:54:59.318 = Long = 1417766099318
Formated date : 01-01-1970 00:00:00.000 = Long = -3600000
Formated date : 01-01-1970 01:00:00.000 = Long = 0
Formated date : 31-11-1969 00:00:00.000 = Long = -2682000000
Formated date : 01-01-2014 00:00:00.000 = Long = 1388530800000
Formated date : 01-01-1970 12:59:59.999 = Long = -1
Formated date : 01-01-1970 01:00:00.000 = Long = 0
Formated date : 01-01-1970 01:00:00.001 = Long = 1
Formated date : 01-01-1970 03:01:00.000 = Long = 7260000
Formated date : 04-12-2014 04:14:44.037 = Long = 1417706084037
Formated date : 31-12-1969 10:59:00.000 = Long = -7260000

Why is:

Formated date : 01-01-1970 01:00:00.000 = Long = 0

This is at 1am. I was expecting 0am.

Answer 1

January 1 1970 at midnight UTC. You need the TimeZone like

public static void main(String[] args) {
    TimeZone tz = TimeZone.getTimeZone("UTC");
    Calendar cal = Calendar.getInstance(tz);
    cal.setTimeInMillis(0);
    DateFormat sdf = new SimpleDateFormat("dd-MM-yyyy hh:mm:ss.SSS");
    sdf.setTimeZone(tz);
    System.out.println(sdf.format(cal.getTime()));
}

Output is

01-01-1970 12:00:00.000

Answer 2

The time represented by the long 0 is midnight 1/1/1970 UTC. This happens to be 1am 1/1/1970 CET. But since you're using a SimpleDateFormat whose timezone is set to CET,

• 1/1/1970 1am is the time that gets displayed when you format the date;
• parsing 1/1/1970 1am gives you the time represented by long 0.

If you had a SimpleDateFormat whose timezone was set to UTC, the behaviour would be entirely different.

You can use the setTimeZone method of DateFormat if you want to use a timezone other than CET.