Time interval sums

How does one sum interval sums?

the following does not work:
select sum(stop-start::interval) as total from startstop
where sid = 545 and
2006 = date_part('year', day) and
9 = date_part('month', day)
however select stop-start::interval) as total from startstop
where sid = 545 and
2006 = date_part('year', day) and
9 = date_part('month', day)
works great but I need an overall sum
Table schema:
Table "public.startstop"
Column | Type | Modifiers
--------+------------------------+-----------
sid | integer | not null
start | time without time zone |
stop | time without time zone |
day | date | not null

thanks

--
Arthur R. Van Hook
Mayor
The City of Lake Lotawana

hook(at)lake-lotawana(dot)mo(dot)us

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