I am working on a project that uses a shared library which uses org.threeten for Date/Time. As the library is shared across many projects it cannot be upgraded to Java 8 yet (sigh). The shortest way seems to be toString()/parse() combination
java.time.OffsetDateTime.parse(org.threeten.bp.OffsetDateTime.toString())
java.time.LocalDate.parse(org.threeten.bp.LocalDate.toString())
But more efficient way in terms of garbage creation will probably be the use of .of(...) methods:
java.time.OffsetDateTime.of(tt.getYear(), tt.getMonth(), tt.getDayOfMonth(), ...)
Have you stumbled upon a library that provides all this sort of conversions between Joda, Threeten and java.time classes?