Calculate dates in Excel and place them in appropriate cell

See attached spreadsheet.? The goal is to calculate propagation and planting dates. I have propagation and planting times in weeks. Based on this, we need to calculate backwards the propagation and planting dates. For example, on the item on line 5 (Adenium), the propagation time is 8 weeks and the planting time 3 weeks, the total time is 11 weeks.? Starting on column O, I have sales information, for Adenium it says 20,000 in April (04/01/2011). So it means that I need to plant them 3 weeks before (21 days) on (03/11/2011). So in the Planting area (starting columns AT) place 20,000 on March (AZ5). Then for the propagation information starting on AB, calculate 8 weeks before (56 days) Januray 14, 2011, so on AX5, we need to the number 20,000 again. I need all this to be calculate automatically. After column AA everything needs to be created by itself.? Thanks! ## Deliverables This program is in Excel 2007. Windows XP.