How to modify decimal fractions from UTC Date Time to Date

I'm trying to convert a utc date time in local date time, but I have some problem the the decimal fraction. I call a web service the return a series of data. One of these data is the utc date time in this format I must the use this library org.threeten.bp, I can't use a different library.

2020-06-22T18:28:57.957535800Z

To converte utcFormat to Date,I have found this piece of code that it works fine

DateFormat utcFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'");
utcFormat.setTimeZone(TimeZone.getTimeZone("UTC"));

Date date = utcFormat.parse("2012-08-15T22:56:02.038Z");

DateFormat pstFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS");
pstFormat.setTimeZone(TimeZone.getTimeZone("ECT"));

System.out.println(pstFormat.format(date));

but it doesen't work well from my code, because return this date

2020-07-03T22:27:52.800

How you can see it's different. I did some test and if I leave only 3 decimal after dot, that part of code it will work fine. Have a look the example:

2020-06-22T18:28:57.800Z

return the right date time from ECT zone

2020-06-22T20:28:57.800

I am looking for a way to receive the utc dateTime with only three decimals or to change the utc dateTime by removing the excess decimals. With this last case I am not if it can be a good idea.

Solution

Here's an approach similar to yours but using classes from org.threeten.bp only instead of mixing it with java.util:

public static void main(String[] args) throws ParseException {
    String datetimeUtc = "2020-06-22T18:28:57.957535800Z";
    // parse it to a ZonedDateTime, this is default formatting ==> no formatter needed
    ZonedDateTime utcTime = ZonedDateTime.parse(datetimeUtc);
    // print the result
    System.out.println(utcTime);
    // convert it to another zone
    ZonedDateTime estTime = utcTime.withZoneSameInstant(ZoneId.of("Europe/Paris"));
    // print that, too
    System.out.println(estTime);
    // define a non-default output format
    DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSS");
    // and print the estTime using that format
    System.out.println(estTime.format(dtf));
}

This outputs the following:

2020-06-22T18:28:57.957535800Z
2020-06-22T20:28:57.957535800+02:00[Europe/Paris]
2020-06-22T20:28:57.957