Depending on whether the start of your buckets needs to be dynamic or fixed, you can move the whole logic into the database:

select 
    -- this is database specific and I don't know the name of the Acce
+ss function
    floor(convert_to_seconds(S_TIME)/600) * 600 as bucket,
    count(*)
group by bucket
[download]

I don't have the Access manual, so I don't know what function you actually need to convert a date/time field to a number of seconds, but I expect a quick search in the help to turn up that link.