I have a large data set which contains from Oct 2002 to Feb 2007. The procedure is: From Oct 19, 2005 to Oct 31, 2005 is week 0.
And after Oct 31, 2005, week is denoted as usual. In other words, Nov 1-7 is week 1, Nov 8-14 is week 2, etc.
And before Oct 19, 2005, week is denoted as from -1. In other words, Oct 18 - 12 is week -1. Oct 11 - 5 is week -2, etc.

How should I do this except manually entering more than 200 lines?