Is it possible to prevent updation of formula based cell ?

Hello ExcelForum experts,

I asked the same question before but didn't get any reply. I have lot of respect for the excel gurus here in the forum and had very good experience in the past. But this time I am really struggling to get an answer to my query. Anyways I am writing it again in the hope that this time I will find the answer.

Actually in my worksheet I calculate the values in a column based on some other cell values. The problem is these referenced cell values may be updated in future dates and if it happens, The values of whole column changed because formula in the column is referencing these cells. I have a date column also.
So I want that if the values of the referenced cells are updated say on 01/09/2013 (dd/mm/yyyy), the values in the column before 01/09/2013 should remain unchanged. Is there any way to achieve this?

I am also attaching an example sheet in which I have made it clear. Plz help me. I need it to be done.

Thanks in advance.