Creating a function in SQL to iterate through dates

What I am attempting to do is create a function of the query below, where instead of using + 45 days, the function iterates through all values from + 35, 36, 37...+45 days and returns the MAX value for EACH cobrand_id.

Notice 45 days appears twice in this query. This would be the input variable.

    with q1 as (
SELECT '17_Q4' qqyy, cobrand_id, sum(calc) AS a
    FROM temp_08.jwn_calc s
where s.optimized_transaction_date > DATE '2016-10-29' AND 
s.optimized_transaction_date <= DATE '2016-10-29' + interval '45 days'
GROUP BY qqyy, cobrand_id )

select q1.qqyy, q1.cobrand_id, q1.a/q2.d AS sum from q1
inner join (
SELECT cobrand_id AS c,
sum(CASE WHEN optimized_transaction_date > date '2015-10-31' 
AND optimized_transaction_date <= date '2015-10-31' + interval '45 days' THEN Calc END) /
sum(CASE WHEN optimized_transaction_date > date '2015-10-31' 
AND 
optimized_transaction_date <= date '2015-10-31' + interval '91 days' THEN Calc END) AS d
FROM temp_08.jwn_calc
GROUP BY cobrand_id
) as q2 on q1.cobrand_id = q2.c

Conceptually, what I am doing is looking at what % of the fiscal quarter was complete 45 days into last year, and using that to extrapolate/divide to derive a full value of this year's quarter.

In response to Generate_series, my question is distinct in that I am querying across 15 different days (35, 36, 37...45 days) to pull out the max value.