Incrementing date in a scheduled request

We have scheduled the Participation Process: Absence to run each evening and, as we wish for the Effective Date parameter to automatically populate with the last day of the current month, we have created a request set, attached the process and chosen a SQL statement for the "Effective Date" parameter of "select fnd_date.date_to_displaydate(last_day(sysdate)) from sys.dual".

However, when this runs, it populates the parameter with the same end of month, even when crossing periods - i.e. if we schedule on the 30 July, it will populate the parameter with the 31st July for both the 30th and 31st, but then will keep the 31st July when it runs on 1 August.