average daily counts to weekly counts

[Dave]

colum a1.a30 contains dates and b1.b30 contains daily counts:

1/1/08 3
1/2/08 5
1/3/08 0
1/4/08 3
....and so on...

What I would like to do with these data is set up a separate adjacent column
like this:
col. d col. e
week 1 avg of 1/1 - 1/7 (or 1/1 - 1/5 -- some range for the week)
week 2 avg of 1/8-1/15
week 3 avg 1/16 - 1/23
.....and so on....

Any way to have these weekly averages caculate (copy the formula down)
without having to adjust the ranges each time?

Thanks