count rows and capture totals while condition is true

Hi,
I am trying to count the number of rows with string starting from "DT" and put the totals in a new colum P. ie P1 and P9 and so on .Below is the Sample data ie In Col A. I need a macro to do this but I:confused need help. Thanks in advance.

HD922222-02
DT01-Feb-11
DT08-Feb-11
DT04-Feb-11
DT22-Feb-11
DT28-Feb-11
DT28-Feb-11
ZZEND
HD923139-02
DT01-Feb-11
DT08-Feb-11
DT15-Feb-11
DT28-Feb-11
DT28-Feb-11
DT28-Feb-11
ZZEND