As offer_date is an number, and is of lower accuracy than your real dates, this may work...
- Convert your real date to a string of format YYYYMM
- Conver that value to an INT
- Compare the result you your offer_date
SELECT
*
FROM
offers
WHERE
offer_date = (SELECT CAST(to_char(create_date, 'YYYYMM') AS INT) FROM customers where id = '12345678')
AND offer_rate > 0
Also, by doing all the manipulation on the create_date you only do the processing on one value.
Additionally, had you manipulated the offer_date you would not be able to utilise any index on that field, and so force SCANs instead of SEEKs.