finding gaps in dates

I have a logging application that should produce an entry in the
database every minute or so, give or take a few seconds.

I'm interested in finding out 
a: what minutes don't have a record and 
b: periods where the gap exceeded a certain amount of time.

The only way I can think of to do it is to create a set returning
function that accepts a begin and end date and returns a
date_trunc('minute', ...) for each minute between then and then doing

Select ts from srf(now() - '30 days'::inteval, now()) where ts not in
(select distinct date_trunc('minute', ts) from table where ts between
now() - '30 days'::inteval and now());

Of course, that seems extremely tedious to me (not tedious to code,
but tedious for the db to process).

If anyone can suggest a better way I'd really appreciate it.

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