SQL & PL/SQL

i have a ccan_dt(connection cancellation date) for every agent

i am trying to calculate the amount the agents spend before 12 months of connection cancellation date ie ccan_dt


agent ccandt
a 14/8/2009
b 12/7/2003
c 08/10/2000
d 7/8/1999


select sum(spend),agent from spendmas where spenddt between d1 and d2 group by agent

in the above d2 is the ccan_dt of each agent and d1 is 12 before date of d2

can any one suggest the efficinet way of acheving this.

i did this by creating a temp table to hold the dates and then calculated the spend.

but how can i achevie this with out creating a temp table.

is there any regular expression which can get 12 months ago date as in OBIEE.

please advice..