You can likely use DateAddCalendarFixedPeriod, then compare.
- Let dueDate = DateAddCalendarFixedPeriod(Today(), "w", 6)
- If $StartDate = dueDate, do something
Though, I will say, Schedule Workflow Rules, in Clarizen, are highly unreliable. The queue/scheduler doesn't work the way you think it works. Likely, to reduce performance impact, items will only appear on the scheduler if they've been "touched" within given period.