Hi Luiz,
do you use the custom workflow for that? If you need to set specific date to a single issue you can just set if manually on full screen issue or on issues list by click on the 'date' field, isn't it?
Anywhere, if you need there is possibility to set the concrete date in workflow by the construction e.g. Due Date = 2013-02-06; , in completion it looks like "yyyy-mm-dd".