ConvertLongToInstantUtil

package com.test;

import java.time.Instant;
import java.time.OffsetDateTime;
import java.time.ZoneId;
import java.time.ZoneOffset;
import java.time.format.DateTimeFormatter;

public class ConvertLongToInstantUtil {
public static void main(String[] args) {
    System.out.println(parse("2018-09-18T19:01:51.821-04:00"));
    System.out.println(parseToInstant(1545955200000L));
    System.out.println(parseInstantToDefaltDateTime(Instant.now()));
  }

  private static String parseInstantToDefaltDateTime(Instant instance) {
    DateTimeFormatter target =
        DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSSZ").withZone(ZoneId.systemDefault());
    return target.format(instance);
  }

  private static OffsetDateTime parseToOffsetDateTime(long instance) {
    return Instant.ofEpochMilli(instance).atOffset(ZoneOffset.ofHours(-4));
  }

  private static Instant parseToInstant(long instance) {
    return Instant.ofEpochMilli(instance);
  }
  
  private static long parse(String instance) {
    OffsetDateTime time = OffsetDateTime.parse(instance);
    return time.toInstant().toEpochMilli();
  }
}