This is what I would like to be able to do:

I want to retain both the old data in a certain Cell and the new data when there is change in the supporting information. The other Cells that rely on this Cell for their calculations decide to use the older data or the newer data based upon today's date. I imagine this might require results to be put into some kind of table with a date stamp everytime the Cell in question changes?

The problem I'm trying to solve is that the results of the calculations in the other Cells cannot change once the time period they are calculating results for has passed. The Cells that are predicting future results, however, can and should change to reflect the new results!