[GENERAL] How do I specify intervals in functions?

[Rob Richardson]

Greetings!
 
In the help file under date and time functions, I see that intervals can
be specified as "interval '3 hours' ".  In a PgAdmin SQL window, I can
enter "select interval '3 hours' ", and it will return me "03:00:00", as
expected.  I can also enter "select '3 hours'::interval", and get the
same result.  Yet neither syntax works inside a function.  
 
declare
     ThreeHours interval;
begin
     ThreeHours = interval '3 hours';  -- throws a syntax error
     ThreeHours = '3 hours'::interval; -- also throws a syntax error
end;
 
So how do I specify an interval in a function?
 
Specifically, I'm trying to do something like the following:
 
    if NewRevisionTime < PredictedEndTime - '08:00:00'::interval then
 
Since both of the shown forms give syntax errors, how do I subtract
eight hours from a time???
 
Thank you very much.
 
RobR, who posted this on the novice list but got no answers.