Is is possible to calculate the no of weeks between one date and 'Today' in a formula field in the CRM?

I have a formula field calculating correctly when comparing two date fields (i.e (((Datecomp(${Vendors.Actual end date},${Vendors.Start date})/60)/24)/7) but I don't want to add another pointless field for today's date.

So ideally it would calculate something like this (((Datecomp('Today',${Vendors.Start date})/60)/24)/7) but that doesn't work.

Any ideas welcome!