Validity Period - Length of Delivery

I have a question about the validity period in the properties of an email campaign delivery activity.  If I am sending an email on 2/27 and don't want anyone to receive the email after that date, would I set the Length of Delivery to 0 days or 1 day?  This field has been described as the length of time that retries are performed, so I'm unsure whether that includes the first day of delivery.