In Java how do I convert from seconds since epoch to milliseconds since January 1, 1970, 00:00:00 GMT

Question

In short:

I have a time that is in epoch and I want to make it time since January 1, 1970, 00:00:00 GMT as java.util.Date would expect to be given.

This code helps demonstrate my issue:

import java.util.Date;
import java.util.Locale;

import org.junit.Test;

import java.text.SimpleDateFormat;
import java.time.LocalDateTime;
import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;

public class TimeHelp {

    private String format = "EEE MMM dd HH:mm:ss yyyy";

    public SimpleDateFormat asSimpleDateFormat() {
        return new SimpleDateFormat(format, Locale.ENGLISH);
    }

    public DateTimeFormatter asDateTimeFormatter() {
        return DateTimeFormatter.ofPattern(format, Locale.ENGLISH);
    }

    @Test
    public void test() throws Exception {
        System.out.println(ZoneId.systemDefault());
        String s = "Sun Apr 04 02:00:01 2010";
        long t1 = asSimpleDateFormat().parse(s).getTime();

        ZonedDateTime zoned = LocalDateTime.parse(s, asDateTimeFormatter())
                    .atZone(ZoneId.systemDefault());
        long t2 = zoned.toEpochSecond() * 1000;

        long t3 = Date.from(zoned.toInstant()).getTime();

        long t4 = zoned.toInstant().toEpochMilli();

        System.out.println("t1 " + t1);
        System.out.println("t2 " + t2);
        System.out.println("t3 " + t3);
        System.out.println("t4 " + t4);
        System.out.println("Difference in minutes " + Math.abs(t1 - t2)/1000/60);
    } 
}

And that outputs:

Australia/Sydney
t1 1270310401000
t2 1270306801000
t3 1270306801000
t4 1270306801000
Difference in minutes 60

Note that t1 is different from all the others, I think because t1 is GMT while the others are all UTC.

If I use the SimpleDateFormat the value of the long is different to if I use the DateTimeFormatter to get a ZonedDateTime after which I call toEpochSecond().

For reasons I would like to be given a ZonedDateTime and I want to convert that to a Date but it looks like such a thing wont work because the Date works in GMT not UTC.

Updated question to include that, note that it gives `1270306801000` not `1270310401000` — Luke, Feb 13 '20 at 02:08
I hope that you are aware that `SimpleDateFormat` is notoriously troublesome and long outdated. You should use `DateTimeFormat` and `ZonedDateTime`. Also consider whether you really need milliseconds since the epoch. If you do, for full precision use `zoned.toInstant().toEpochMilli()`. — Ole V.V., Feb 13 '20 at 03:54

score 2 · Accepted Answer · answered Feb 13 '20 at 02:24

Quoting timeanddate.com:

When local daylight time was about to reach Sunday, April 4, 2010, 3:00:00 am clocks were turned backward 1 hour to Sunday, April 4, 2010, 2:00:00 am local standard time instead.

Which means that Sun Apr 04 02:00:01 2010 happened twice that day. So which of those 2 do you get?

With SimpleDateFormat you get the later one, although that is undocumented.

With ZonedDateTime you get the earlier one:

For Overlaps, the general strategy is that if the local date-time falls in the middle of an Overlap, then the previous offset will be retained. If there is no previous offset, or the previous offset is invalid, then the earlier offset is used, typically "summer" time.. Two additional methods, withEarlierOffsetAtOverlap() and withLaterOffsetAtOverlap(), help manage the case of an overlap.

In Java how do I convert from seconds since epoch to milliseconds since January 1, 1970, 00:00:00 GMT

1 Answers1