In joda time how to convert time zone without changing time

If your timestamp is: 2015-01-01T00:00:00.000-0500 (this is local time [for me])

Try this:

DateTime localDt = new DateTime(timestamp.getTime())
    .withZoneRetainFields(DateTimeZone.UTC)
    .withZone(DateTimeZone.getDefault());

2014-12-31T19:00:00.000-05:00

Breaking it down: This gives you a DateTime corresponding to your timestamp, specifying that it is in UTC:

new DateTime(timestamp.getTime())
    .withZoneRetainFields(DateTimeZone.UTC)

2015-01-01T00:00:00.000Z

This gives you a DateTime but with the time converted to your local time:

new DateTime(timestamp.getTime())
    .withZoneRetainFields(DateTimeZone.UTC)
    .withZone(DateTimeZone.getDefault());

2014-12-31T19:00:00.000-05:00

You can use the withZoneRetainFields() method of DateTime to alter the timezone without altering the numerals in the date.

You can use class LocalDateTime

LocalDateTime dt = new LocalDateTime(t.getTime()); 

and convert LocalDateTime to DateTime

DateTime dt = new LocalDateTime(timestamp.getTime()).toDateTime(DateTimeZone.UTC);  

Joda DateTime treats any time in millis like "millis since 1970y in current time zone". So, when you create DateTime instance, it is created with current time zone.