Just wondering, thanking you all kindly! ^-^
When you post the poll, there's a field to add an expiration under the poll options:
Going to check and see if you can add this to a poll that's already been created.
Looks like if you click 'Edit Poll', which I think anybody can do for their own poll if they posted it, you can then populate the expiration date. You can also tell it to keep the results hidden until the expiration date if you want! (I've hardly used the poll feature before, so I didn't realize what settings existed >.<)