Andrew,
I believe I have found the issue and also believe it is a bug in v6 Beta 2. The errors i found in the catalina.out log look like this:
ERROR 09 Jul 2017 01:00:08 Scripting - JavaScriptEvaluator -> The result of expression var d = new java.text.SimpleDateFormat('dd/MM/yyyy'); d.parse(17/07/2017); can't be evaluated - error message=Wrapped java.text.ParseException: Unparseable date: "0.0012040512784191515" (#1)
ERROR 09 Jul 2017 01:00:08 Shark - JavaScriptEvaluator -> The result of expression var d = new java.text.SimpleDateFormat('dd/MM/yyyy'); d.parse(17/07/2017); can't be evaluated - error message=Wrapped java.text.ParseException: Unparseable date: "0.0012040512784191515" (#1)
ERROR 09 Jul 2017 01:00:08 org.joget.workflow.model.service.WorkflowManagerImpl - org.mozilla.javascript.WrappedException: Wrapped java.text.ParseException: Unparseable date: "0.0012040512784191515" (#1)
org.mozilla.javascript.WrappedException: Wrapped java.text.ParseException: Unparseable date: "0.0012040512784191515" (#1)
However, when I put quotes around the Hash Variable in the Deadline specification it began to work properly.

According to the documentation quotes should not be required so this may be an omission in the code.
Thanks, Bill