How to find number of seconds between 2 timestamps

I am trying to calculate a rate per second and am having trouble getting the
number of seconds between the two timestamps to use in the formula.

overview=> select extract(epoch from interval '1 day'::interval); date_part
-----------    86400
(1 row)
overview=> select extract(epoch from interval ('2009-02-16
22:15:28.034567-06'::timestamp with time zone - '2009-02-15
22:15:28.034567-06'::timestamp with time zone));
ERROR:  syntax error at or near "'2009-02-16 22:15:28.034567-06'"
LINE 1: select extract(epoch from interval ('2009-02-16 22:15:28.034...

Any suggestions would be helpful,

Thanks,
Woody

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