Dear all,

I am quite new with stata, and after a couple of days trying to fix a problem without any success I decided to post my problem here.
Basically, I have a panel dataset where permno identifies the companies, and BM indicates the book to market value of each company which is calculated in December of each year.
In accordance with my purpose, for each company the BM for month t from June of year t+1 through May of the year t+2 is taken to be the book to market ratio calculated in December of year t.
Therefore I am not able to replace the BM value calculated in December of year t in June of year t+1 through May t+2.
This replacement should be done for each company as well.
Any help would be really appreciated.
Kind regards
permno date year month BM
10001 30-Apr-87 1987 4
10001 29-May-87 1987 5
10001 30-Jun-87 1987 6
10001 31-Jul-87 1987 7
10001 31-Aug-87 1987 8
10001 30-Sep-87 1987 9
10001 30-Oct-87 1987 10
10001 30-Nov-87 1987 11
10001 31-Dec-87 1987 12 1.207618
10001 29-Jan-88 1988 1
10001 29-Feb-88 1988 2
10001 31-Mar-88 1988 3
10001 29-Apr-88 1988 4
10001 31-May-88 1988 5
10001 30-Jun-88 1988 6
10001 29-Jul-88 1988 7
10001 31-Aug-88 1988 8
10001 30-Sep-88 1988 9
10001 31-Oct-88 1988 10
10001 30-Nov-88 1988 11
10001 30-Dec-88 1988 12 1.145192
10001 31-Jan-89 1989 1
10001 28-Feb-89 1989 2
10001 31-Mar-89 1989 3
10001 28-Apr-89 1989 4
10001 31-May-89 1989 5