Before you deploy your poll for review, you can change the poll’s start and end dates in its Settings tab, to make the poll end earlier or later.
Once the poll is Active, but before it ends, you can come back to the Settings tab and click the Extend Voting Period button. You can pick any date past the current end date, and you can choose whether to email all voters who have not yet participated in the poll to tell them about the extension.
BallotBox does not permit you to shorten the voting period, however. Voters may be relying on the end date you previously set; for example, a voter may have seen the email about your poll, and determined that she can wait until after she’s back from a vacation to cast her ballot. It would be unfair to some voters if you were to shorten the poll’s voting period.