As I think about this more, your apparent requirement is that everytime you cross Sunday you need to add an additional day, correct?

Example, if the startdate is 2012-11-30 (a Friday) and you add 3 days you would normally get 2012-12-03 (a Monday). What you want, however, is to get 2012-12-04 (a Tuesday). Is this correct?

Edit: And you probably want this to work over a year boundary as well, correct?