Certifying a check is an antiquated service that banks are allowed, but not required to offer. Very few do. If you offer the service, you may offer it on a case by case basis.
A certified check is a direct obligation of the bank; it is the practical but not the legal equivalent of a cashiers check. The concept of stale dating does not apply.
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.