You’ll need to download the jar from here, and specify where it is in the “Additional Libraries” tab of the Java Snippet node… otherwise this should work out of the box for you.
This workflow is large because it contains the data.
islamic date conversion.knwf (2.1 MB)
If your date column format changes, or you need to use a different leap year pattern, look at the code in the Java Snippet panel - i put comments in it as to where these are defined.