Difference between two times as a numeric value in a stored procedure.

Hello List,

I am writing two stored procedure which alternatively returns the dayhours and nighthours of two times. (nighthours are
consideredbetween 00:00 and 06:00). 

As an example here is the getdayhours function:
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CREATE OR REPLACE FUNCTION public.getdayhours(time, time) RETURNS interval AS
'DECLARE begintime ALIAS FOR $1; endtime ALIAS FOR $2; begindate timestamp; enddate timestamp; tmpresult interval;
BEGIN IF endtime = time \'00:00\' THEN    enddate := (current_date+1)+endtime; ELSE     enddate :=
current_date+endtime;END IF; IF begintime < time \'06:00\' THEN   begindate := current_date + time \'06:00\'; ELSE
begindate:= current_date+begintime; END IF; tmpresult := enddate-begindate; IF tmpresult<\'00:00\' THEN    return
\'00:00\';ELSE   return tmpresult; END IF; 
END;' LANGUAGE 'plpgsql' VOLATILE;
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The working of the functions is not the problem, but the return type is. I can't seem to find a way to substract two
timevalues (or timestamp values) and get a numeric/float value. I always get the INTERVAL datatype. For example, in
steadof 4:30 i would like 4.5 as a result. I have searched the documentation but could not find any way to substract
time/timestampvalues and get a numeric/float as a result. When I try to CAST the interval to a numeric or float value I
getan error (cannot cast time without tz to ...). Same goes for trying to cast the beginvalues and then substract them.
Doesanyone have any idea how I can solve/circumvent this problem? Is there a function I can use? 

I don't know if it helps but I'm going to use the functions like this:
SELECT workhour_id, employee_id, task_id, whdate, begintime, endtime, getdayhours(begintime,endtime),
getnighthours(begintime,endtime)FROM workhour 


Thanks in advance.

Stijn Vanroye