Workflow Rules for Alerts

Hi all,

I would like to be advised when one of my clients' account expires.
But the choice is limited : I can create an alert so as to be informed 1 day earlier only.
Is it possible to change the formula (---> ${TOMMOROW} ) into more precise terms ( one or two weeks earlier i.e) ?
I already tried to change the due date (plus/minus X days but it does not seem to work)

Thanks in advance if you can give me some clue here.