How can I set up a "conditional" copy and paste procedure?

I need to copy certain values from a column and paste them into another
depending on the date (that is settled in another column) associated to them.

Example:

ColumnA        ColumnB                  ColumnC
Date         Delay(nº of days)       Where to paste

13-01-06             15                         ........
22-02-05             20                         ........
24-01-06             30                         ........
 
So..., I need to copy the values in Column B to the Column C if the
corresponding values in Column A are greater than 01-01-2006 for example?

Taking into account that I have an huge list in which I'm adding new rows
every now an then, I would like to find a way that every time I added a new
row, depending on the values in column A it would copy the values in column B
and paste them to the column C.

I could really use some help!

Many Thanx!