Use expression as process duedate value

When configured expression as process duedate value as:
<timer duedate="#

"/>
and started process like:
Map<String, String> variables = new HashMap<String, String>();
variables.put("process_duedate", "24 business minutes");
ProcessInstance processInstance = executionService.startProcessInstanceByKey
("TimerRepeat", variables);

but engine reported :
"javax.el.PropertyNotFoundException: Cannot find property process_duedate"

The details on this issue, please reference to discussion named "Using expression as
process duedate value" and the link is:
https://community.jboss.org/thread/153992