Simplest way to get local milliseconds in a time zone with Joda-Time
long localMillis = dateTime.withZone(timeZone).toLocalDateTime()
    .toDateTime(DateTimeZone.UTC).getMillis();

private static final JAN_1_1970 = new LocalDateTime(1970, 1, 1, 0, 0);

...

new Duration(JAN_1_1970.toDateTime(someTimeZone), endPointInstantOrDateTime).getMillis();