Yet another newbie

[Mario.A.C]

Hi to all of you
I've got a doubt that is driving me crazy;
I'm running for the first time Bind 8.2.2 P5 under Linux , I've noticed that
one of the "fields" in the soa record is called Expire Time ; now I'm
wondering , what does happens when this time is finished? the zone is
removed? Bind ignores that zone so a request will end in a failure?
and how can I automatically upgrade serial numbers of zones that will never
change?

thanks for your time
regards
Mario