Track & Record Dates, Past And Actual

I am creating a tracker to record, incoming, latest date of reply needed and actual date of reply. So I have column A= received, Column B = need to reply by, Column C = replied on. What I want in A is today’s date which is easy enough with ctrl+; is there a way of having it set that as soon as the cell is clicked on that date is activated and until then it remains empty?

What I have in B is A# + 20 this adds 20 days to the date entered in column A, again is there any way to have it set that this automatically activates when cells in column A are active and not until then, as if I enter that code in the cell and nothing is showing in cell A it gives a date of the 20/01/2008.

In C I have tried a couple of codes and they have not worked so what I would like is a code that when A and B are active it generates a countdown for days left to reply and if this countdown reaches zero then the whole line is flagged up in red as late. Lastly, is there a code that I can use that will automatically correspond with the A, B or C columns and match them with the same number cell in the other columns, sorry this isn’t so easy to explain but what I have to do at the moment is type in B is =A1+20 , A2+20, A3+20 and so on rather than just typing in A……+20 for column B to know it needs to associate itself with the same number cell in column A regardless what number it is.