I've finally gotten around to adding extension methods to the Groovy JDK for the java.time types. See https://issues.apache.org/jira/browse/GROOVY-8334 for reference.
The additions are in the java.time and java.time.temporal packages, obviously, as well as some "since 3.0" methods in java.util.Date and java.util.Calendar
I thought it was worthwhile to open up these API changes for discussion here before submitting a PR. There may be things included which should not be or things not included that should.