“This gift certificate is valid for 18 months, followed by a 6-month grace period, after which it will expire.”

It would be efficient, technical, and correct to say “24 months” or “two years”.

However, it wouldn’t be more human.

By giving a grace period with their gift certificates, Amazon.com is saying, “you’d better shop soon,” but, “hey, if you forget, we’re adding some extra time for you.”

Amazon.com, again, shows you don’t need a geriatric greeter to make your shopping experience more human.