Is the subpoena valid only for the date and time on the subpoena that was served?

Not sure what is being asked - - they can be delivered out-of-state (for instance) by registered mail. They can be delivered verbally face-to-face via a law enforcement officer or Bailiff. (???)

Yes, as long as you are notified of the change of date.

Check and see what the subpoena says about a time limit. Usually you are under subpoena for as long as the case lasts, and required to update as more responsive material comes...

Anywhere the subject is found, or can be located.

In many courts a summons may be served by mail under certain conditions, but a subpoena must be served personally. Whether any particular court allows service by mail will be found by reading that...