Calculating future date

At my work we need to calculate when a patient is to come back to the office for a visit. There is also a window included in this. For instance:

Patient is seen on day one. I would like to enter that date in excel and get the following:

Week 4 with a window of + or - 3days
Week 16 with a window of + or - 7days
Week 24 with a window of + or - 7days

These values vary from time to time and would have to be changed. For instance the next time I may need to enter day one and get 5 more visits: day 3 (no window), day 8 (+3day window), day 15 (+ or -5 day window), day 28 and day 57 (+ or - 7day window)

The outcome fields would show the exact date that the person is to return with the next column showing the window. See example below:

I would enter: Friday Sept. 11, 2009

I would get:

Day 3 _________9/13 ___________ no window
Day 8 _________9/18 ___________ 9/18 - 9/21
Day 15 ________9/25 ___________ 9/20 - 9/30
Day 29 ________10/9 ___________ 10/2 - 10/16
Day 57 ________11/6 ___________ 10/30 - 11/13