Grouping by a date period.

[R A]

Let's say I have a table with dates that range from January to May in each record.
Is it possible to group the records by a date range, for example:

I want to group the records by a date period of 1 month, starting at the 3rd day of each month. So what I'd want to see is, five records for each month (jan 3 to feb 3, feb 3 to march 3, march 3 to april 3, april 3 to may 3).

At the moment I have the following:

select id, _date, sum(cost) from test
where
_date >= ' 2005-01-03' and
_date <= date_add('2005-01-03', interval 1 month)
group by extension
union
select extension, _date, sum(cost) from test
where
_date >= date_add(' 2005-01-03', interval 1 month) and
_date <= date_add('2005-01-03', interval 2 month)
group by extension

The trouble with this is that each month interval has its own select query and that the increase in the month is hard coded (interval 1 month for example). Is it possible to use SQL to inrement the month?

Thanks in advance ... (hope this made sense).