Adding to a date

I was beginning to create some financial functions for calculating
Future Values, PV's, etc -for basic retirement planning.
example:

CREATE OR REPLACE FUNCTION lotsa.FVPMT(payment double precision,
interestRate double precision, periods double precision)
RETURNS double precision AS
$BODY$
BEGIN
return payment*(pow(1+interestRate, periods)-1)/interestRate;
END;
$BODY$
LANGUAGE 'plpgsql' VOLATILE

periods seems to be a problem. Usually, I'd be getting birth_date
from a file, but I'm not finding a way to
add 65 years - or 67 or whatever, to get the period between now (or an
assumed date) and retirement.

I'm hoping there is a way to:
1) Date of birth + xx years - with a result of date (Result 1)
2) Subtract now or an assumed date from Result 1 to get Result 2
3) Divide by ~ 365.25 for years, which would be 'periods' in the function.

I've gotten around it, but it sure seems pretty messy:
Greatest(FVPMT(test_fv.pmt, test_fv.i_rate,
(23741.25-(test_fv.start_date-test_fv.dob))/365.25),0)

where the 23741.25 is 65* 365.25.

Any help would be appreciated. I've read all of what would seem to be
appropriate.
Doesn't mean I understood it, but I read it.

I'm not concerned about being off by a day or 2.
Small potatoes compared to the variations rate of return will undergo.

Thanks