Convert Unix epoch time

I am trying to convert Unix epoch time to an ISO 8601 (https://en.wikipedia.org/wiki/ISO_8601) with the time zone. I have it working but only if it's local time. How can I pass in the time zone as an offset?

Input 1: 1483772400000

Input 2: -0700 (but I can convert as needed)

Desired output: 2017-01-07T00:00:00-07:00 or simply 2017-01-07T00:00:00-07

Current output: 2017-01-06 23:00:00-08 as I'm in PST

I'm using IntelliJ IDEA, JDK 1.8u202 .

    package util.date;
    
    import java.util.Date;
    import java.util.Locale;
    import java.util.TimeZone;
    import java.text.SimpleDateFormat;
    
    public class EpochToISO {
        public String epochToIso8601(long time) {
            String format = "yyyy-MM-dd HH:mm:ssX";
            SimpleDateFormat sdf = new SimpleDateFormat(format, Locale.getDefault());
            sdf.setTimeZone(TimeZone.getDefault());
            return sdf.format(new Date(time));
        }
    
        public static void main(String[] args) {
            EpochToISO convert = new EpochToISO();
            long epoch = 1483772400000; // Example
            String iso8601 = convert.epochToIso8601(epoch);
            System.out.println(epoch + " -> " + iso8601);
        }
    }

Copyright Notice：Content Author:「Ryan B」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/73710571/convert-unix-epoch-time