Calculate the sum of a field filtered by a window defined on another field

I have table event:

event_date,
num_events,
site_id

I can easily use aggregate SQL to do SELECT SUM(num_events) GROUP BY site_id.

But I also have another table site:

site_id,
target_date

I'd like to do a JOIN, showing the SUM of num_events within 60 days of the target_date, 90 days, 120 days, etc. I thought this could easily be done using a WHERE clause in the aggregate SQL. However, this is complicated by two challenges:

1. The target_date is not fixed, but varies for each site_id
2. I'd like multiple date ranges to be output in the same table; so I can't do a simple WHERE to exclude records falling outside the range from the event table.

One workaround I've thought of is to simply make several queries, one for each date range, and then use a view to paste them together. Is there a simpler, better, or more elegant way to achieve my goals?

You would do something like:

select sum(case when target_date - event_date < 30 then 1 else 0 end) as within_030,
       sum(case when target_date - event_date < 60 then 1 else 0 end) as within_060,
       sum(case when target_date - event_date < 90 then 1 else 0 end) as within_090    
from event e join
     site s
     on e.site_id = s.site_id;

That is, you can use conditional aggregation. I am not sure what "within 60" days means. This gives days before the target date, but similar logic will work for what you need.