You haven't specified which type within Joda Time you're interested in, but:
Instant instant = ...;
Date date = instant.toDate();
instant = new Instant(date);
// Or...
instant = new Instant(date.getTime());
Neither Date
nor Instant
are related to time zones, so there's no need to specify one here.
It doesn't make sense to convert from LocalDateTime
/ LocalDate
/ LocalTime
to Date
(or vice versa) as that would depend on the time zone being applied.
With DateTime
you can convert to a Date
without specifying the time zone, but to convert from Date
to DateTime
you should specify the time zone, or it will use the system default time zone. (If you really want that, I'd specify it explicitly to make it clear that it's a deliberate choice.)
For example:
DateTimeZone zone = DateTimeZone.forID("Europe/London");
Date date = ...;
DateTime dateTime = new DateTime(date.getTime(), zone);