Summing based on >=date

I am creating a running targeting program for the staff at work and am stuck on running a progressive tally.

Ill explain what I have first.

A1=TODAY()-DAY(TODAY())+1), A2==TODAY()-DAY(TODAY())+1 A3=(A1+7), A4=(A2+7)... gives me a progressive date log of 7 days.

Column B is the names of the salesperson. (pretty well irrelevant for this question.)

C1=Total sales (This will be updated every day automatically from a generated report using a data link)

What I need is for sales to copy from C1(TOTAL SALES) to C2, this is easy enough. Once the first 7 days have gone, I need to start summing up the sales from the 8th day onwards and put into C3. So if A3 is greater than or equal to A1, I need C3 to equal sales over and above C2.
eg. Between the 1st and 7th we sell $10000, C2 = $10000. From the 8th to the 14th, we sell another $5000. I need C2 to show $10000, and C3 to show $5000. From the 15th to the 21st we sell $7000. C2=$10000, C3 = $5000, C4=$7000, C1 =$22000.

I hope this makes sense, really hard to put into words. Thank you so much in advance