Need Assistnace In Creating SQL to Get Data on a Quarterly Basis

 Hi Folks

looking for a little direction in trying to come up with some sql that will give me totals quarterly given a table of data

here is what i have for generating by year and by month, but i would like to generate the data quarterly:

input:

select  
   TO_CHAR (createdate,'yyyy/mm') monthofyear,
   COUNT (createdate) accountscreated
from  
   dtree
where  
   subtype=142
group by
   TO_CHAR (createdate,'yyyy/mm')
order by
   TO_CHAR (createdate,'yyyy/mm');

sample output:

Month of Year       Accounts Created
---------------- -------------------
2004/11                           89
2004/12                          207
2005/01                          112
2005/02                          322
2005/03                          185
2005/04                          163
2005/05                          213
2005/06                          164

etc, etc

any sql gurus have any handy dandy tricks ?

Thaks

jim