Convert timestamp to a specified timezone offset - timezone

Question

How can I convert a long timestamp, e.g. 2556072000000L, to a String in a specified timezone offset, e.g. +08:30? I looked into Joda methods but didn't find a solution.
Similarly, how can I convert a String timestamp, e.g. 2050-12-31T04:00:00Z, to a String in a specified timezone offset, e.g. +08:30?
2050-12-31T12:30:00+08:30 in the specified offset is expected.
Parsing +08:30 into three parts: +, hour offset (8) and minute offset (30), add the offset to the timestamp or dateTime could be a solution. However, I'm wondering whether there's already a solution so that I needn't reinvent wheels?
/*
* timestamp: 2556072000000L (2050-12-31T04:00:00Z)
* offset: +08:30
* return: 2050-12-31T12:30:00+08:30
*/
String toOffset(long timestamp, String offset);
/*
* dateTime: 2050-12-31T04:00:00Z
* offset: +08:30
* return: 2050-12-31T12:30:00+08:30
*/
String toOffset(String dateTime, String offset);

Answer 1

Reading this article, I got the answer to my question. Here it is:
import java.time.ZoneOffset;
/*
* timestamp: 2556072000000L (2050-12-31T04:00:00Z)
* offset: +08:30
* return: 2050-12-31T12:30:00+08:30
*/
public String toOffset(long timestamp, String offset); { // +08:30
DateTime time = new DateTime(timestamp + getOffsetMillis(offset));
time = time.withZone(DateTimeZone.forID(DateTimeZone.UTC.getID())); // 2050-12-31T12:30:00Z
DateTimeFormatter formatter = DateTimeFormat.forPattern("yyyy-MM-dd'T'HH:mm:ss");
String dateStr = time.toString(formatter); // 2050-12-31T12:30:00
return dateStr + offset; // 2050-12-31T12:30:00+08:30
}
public long getOffsetMillis(String offset) { // +08:30
ZoneOffset zoneOffSet = ZoneOffset.of(offset);
return zoneOffSet.getTotalSeconds() * 1000; // (8 * 60 * 60 + 30 * 60) * 1000
}
Please note that the timezone offset must be in the inclusive range of [-18:00, +18:00]. Below is quoted from JDK document
In 2008, time-zone offsets around the world extended from -12:00 to
+14:00. To prevent any problems with that range being extended, yet still provide validation, the range of offsets is restricted to -18:00
to 18:00 inclusive.
If you look at the Joda timezone table, you will find that offsets for all the timezones are in the range of [-12:00, +14:00].